Cell surface conjugates and related cell compositions and methods

Abstract

Provided herein are cell surface conjugates containing a cell surface molecule and at least one agent, such as at least one affinity tag, and engineered cells expressing such cell surface conjugates. In some embodiments, the cell surface molecule does not contain an intracellular signaling domain or is not capable of mediating intracellular signaling. In some embodiments, the cells engineered to contain the cell surface conjugate, such as T cells, further contain a genetically engineered recombinant receptor that specifically binds to antigens, such as a chimeric antigen receptor (CAR). Also provided are methods of detecting, identifying, selecting or targeting cells expressing the cell surface conjugates, such as in connection with methods of manufacturing engineered cells or in connection with administration of such cells to subjects, including methods of adoptive cell therapy.

1 . A cell surface conjugate, comprising:
 (a) a cell surface molecule that lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling; and   (b) at least one agent linked to the cell surface molecule, the agent being capable of binding a streptavidin, a streptavidin analog or a streptavidin mutein.   
     
     
         2 . The cell surface conjugate of  claim 1 , wherein the agent exhibits a binding affinity for streptavidin or a streptavidin mutein with an equilibrium dissociation constant (K D ) of from or from about 10 −4  M to or to about 10 −10  M. 
     
     
         3 . A cell surface conjugate, comprising:
 (a) a cell surface molecule that lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling; and   (b) at least one agent linked to the cell surface molecule and being capable of reversibly binding to a reagent and/or capable of being competed in the presence of a competition substance, wherein the agent is a peptide of less than 50 amino acids in length.   
     
     
         4 . The cell surface conjugate of  claim 3 , wherein the agent exhibits a binding affinity for the reagent with an equilibrium dissociation constant (K D ) of from or from about 10 −4  M to or to about 10 −10  M. 
     
     
         5 . The cell surface conjugate of  claim 3  or  claim 4 , wherein the reagent is a streptavidin, a streptavidin analog or a streptavidin mutein. 
     
     
         6 . A cell surface conjugate, comprising:
 (a) a cell surface molecule that lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling; and   (b) at least one agent linked to the cell surface molecule, the agent having a binding affinity for a reagent with an equilibrium dissociation constant (K D ) of more than 10 −7  M or an equilibrium association constant (K A ) of less than 10 7  M −1 .   
     
     
         7 . The cell surface conjugate of  claim 6 , wherein the reagent is a streptavidin, a streptavidin analog or a streptavidin mutein. 
     
     
         8 . The cell surface conjugate of any of  claims 1 - 7 , wherein the cell surface molecule comprises a transmembrane domain and/or is capable of being expressed on the surface of the cell. 
     
     
         9 . The cell surface conjugate of any of  claims 1 - 8 , wherein the cell surface molecule is modified compared to a reference cell surface molecule, optionally wherein the reference cell surface molecule is a cell surface receptor comprising an intracellular signaling domain. 
     
     
         10 . The cell surface conjugate of  claim 9 , wherein the modified cell surface molecule exhibits altered cellular internalization, enzymatic activity and/or ligand binding, compared to the reference cell surface molecule. 
     
     
         11 . A cell surface conjugate, comprising:
 (a) a cell surface molecule that is modified compared to a reference cell surface molecule, wherein the modified cell surface molecule exhibits altered cellular internalization, enzymatic activity and/or ligand binding, compared to the reference cell surface molecule; and   (b) at least one agent linked to the cell surface molecule, the agent being capable of binding a streptavidin, a streptavidin analog or a streptavidin mutein.   
     
     
         12 . The cell surface conjugate of  claim 11 , wherein the cell surface molecule lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling. 
     
     
         13 . A cell surface conjugate, comprising:
 (a) a cell surface molecule comprising a prostate-specific membrane antigen (PSMA) or a modified cell surface molecule thereof; and   (b) at least one agent linked to the cell surface molecule, the agent being capable of binding a streptavidin, a streptavidin analog or a streptavidin mutein.   
     
     
         14 . The cell surface conjugate of  claim 13 , wherein:
 the modified cell surface molecule lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling; and/or   the modified cell surface molecule is modified compared to a reference cell surface molecule, wherein the modified cell surface molecule exhibits altered cellular internalization, enzymatic activity and/or ligand binding, compared to the reference cell surface molecule.   
     
     
         15 . The cell surface conjugate of any of  claims 11 - 14 , wherein the cell surface molecule comprises a transmembrane domain and/or is capable of being expressed on the surface of the cell. 
     
     
         16 . The cell surface conjugate of any of  claims 11 - 15 , wherein the agent exhibits a binding affinity for a streptavidin, a streptavidin analog or a streptavidin mutein with an equilibrium dissociation constant (K D ) of from or from about 10 −4  M to or to about 10 −10  M. 
     
     
         17 . The cell surface conjugate of any of  claims 1 - 16 , wherein the binding of the agent to the reagent is reversible and/or capable of being competed in the presence of a competition substance. 
     
     
         18 . The cell surface conjugate of  claim 17 , wherein the competition substance exhibits a higher binding affinity for the reagent than the binding affinity of the agent for the reagent. 
     
     
         19 . The cell surface conjugate of  claim 18 , wherein:
 the competition substance exhibits a binding affinity for the reagent with an equilibrium dissociation constant (K D ) of between or about between 10 −10  M and 10 −14  M; and/or   the agent exhibits a binding affinity for the reagent with an equilibrium dissociation constant (K D ) of more than 10 −10  M.   
     
     
         20 . The cell surface conjugate of any of  claims 1 ,  2 ,  5 ,  7 - 20 , wherein the binding of the agent to the streptavidin, streptavidin analog or streptavidin mutein is reversible and/or capable of being competed in the presence of biotin, a biotin analog or a biologically active fragment thereof. 
     
     
         21 . The cell surface conjugate of any of  claims 1 - 20 , wherein the at least one agent is linked directly to the cell surface molecule. 
     
     
         22 . The cell surface conjugate of any of  claims 1 - 20 , wherein the at least one agent is linked indirectly to the cell surface molecule via at least one linker. 
     
     
         23 . The cell surface conjugate of any of  claims 1 - 22 , wherein the at least one agent comprises from or from about 1 to 4 or 1 to 2 agents. 
     
     
         24 . The cell surface conjugate of any of  claims 1 - 23 , wherein the at least one agent comprises only one agent. 
     
     
         25 . The cell surface conjugate of any of  claims 1 - 24 , wherein the agent is linked to an extracellular portion or region of the cell surface molecule, optionally wherein the extracellular portion or region is at the N-terminus or C-terminus of the cell surface molecule. 
     
     
         26 . The cell surface conjugate of any of  claims 1 - 25 , wherein the agent is linked at the N-terminus of the cell surface molecule. 
     
     
         27 . The cell surface conjugate of any of  claims 1 - 26 , wherein the agent is linked at the C-terminus of the cell surface molecule. 
     
     
         28 . A cell surface conjugate, comprising a cell surface molecule linked, at an extracellular portion or region of the cell surface molecule, to an agent, the agent being capable of binding a reagent that is or comprises streptavidin or a streptavidin mutein, optionally wherein the extracellular portion or region is at the N-terminus or C-terminus of the cell surface molecule. 
     
     
         29 . A cell surface conjugate, comprising a cell surface molecule linked, at an extracellular portion or region of the cell surface molecule, to an agent, the agent being capable of reversibly binding to a reagent, wherein the agent is a peptide of less than 50 amino acids in length optionally wherein the extracellular portion or region is at the N-terminus or C-terminus of the cell surface molecule. 
     
     
         30 . The cell surface conjugate of  claim 28  or  claim 29 , wherein the agent exhibits a binding affinity with an equilibrium dissociation constant (K D ) of from or from about 10 −4  M to or to about 10 −10  M. 
     
     
         31 . A cell surface conjugate, comprising a cell surface molecule linked, at an extracellular portion or region of the cell surface molecule, to an agent, wherein the agent exhibits a binding affinity for a reagent with an equilibrium dissociation constant (K D ) of more than 10 −7  M or an equilibrium association constant (K A ) of less than 10 7  M −1  optionally wherein the extracellular portion or region is at the N-terminus or C-terminus of the cell surface molecule. 
     
     
         32 . The cell surface conjugate of any of  claims 28 - 31 , wherein the agent is linked at the N-terminus of the cell surface molecule. 
     
     
         33 . The cell surface conjugate of any of  claims 28 - 31 , wherein the agent is linked at the C-terminus of the cell surface molecule. 
     
     
         34 . The cell surface conjugate of any of  claims 28 - 33 , wherein the reagent is or comprises a streptavidin, a streptavidin analog or a streptavidin mutein. 
     
     
         35 . The cell surface conjugate of any of  claims 28 - 34 , wherein the binding of the agent to the reagent is reversible and/or capable of being competed in the presence of a competition substance. 
     
     
         36 . The cell surface conjugate of  claim 35 , wherein the competition substance exhibits a higher binding affinity for the reagent than the binding affinity of the agent for the reagent. 
     
     
         37 . The cell surface conjugate of  claim 36 , wherein:
 the competition substance exhibits a binding affinity for the reagent with an equilibrium dissociation constant (K D ) of between or about between 10 −10  M and 10 −14  M; and/or   the agent exhibits a binding affinity for the reagent with an equilibrium dissociation constant (K D ) of more than 10 −10  M.   
     
     
         38 . The cell surface conjugate of any of  claims 28 ,  34 - 37 , wherein the binding of the agent to the streptavidin, streptavidin analog or streptavidin mutein is reversible and/or capable of being competed in the presence of biotin or a biotin analog. 
     
     
         39 . The cell surface conjugate of any of  claims 28 - 38 , wherein the agent is linked directly to the cell surface molecule. 
     
     
         40 . The cell surface conjugate of any of  claims 28 - 38 , wherein the agent is linked indirectly to the cell surface molecule via at least one linker. 
     
     
         41 . The cell surface conjugate of any of  claims 28 - 40 , wherein the cell surface molecule is linked to only one agent. 
     
     
         42 . The cell surface conjugate of any of  claims 1 - 41 , wherein the cell surface molecule is not a chimeric antigen receptor (CAR). 
     
     
         43 . The cell surface conjugate of any of  claims 28 - 30 , wherein the cell surface molecule is modified compared to a reference cell surface molecule. 
     
     
         44 . The cell surface conjugate of  claim 43 , wherein the modified cell surface molecule lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling; and/or the modified cell surface molecule exhibits altered cellular internalization, enzymatic activity and/or ligand binding, compared to the reference cell surface molecule. 
     
     
         45 . The cell surface conjugate of  claim 43  or  claim 44 , wherein the reference cell surface molecule is a native mammalian cell surface molecule. 
     
     
         46 . The cell surface conjugate of any of  claims 1 - 45 , wherein the cell surface molecule comprises an epitope capable of being recognized by an antibody or antigen-binding fragment thereof. 
     
     
         47 . The cell surface conjugate of any of  claims 1 - 33  that is a fusion protein. 
     
     
         48 . The cell surface conjugate of any of  claims 1 ,  2 ,  5 ,  7 - 28  and  34 - 47 , wherein the streptavidin analog or mutein comprises the amino acid sequence Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  at sequence positions corresponding to positions 44 to 47 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         49 . The cell surface conjugate of any of  claims 1 ,  2 ,  5 ,  7 - 28  and  34 - 48 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in any of SEQ ID NOS: 3-6, 27 and 28; 
 b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to any of SEQ ID NOS:3-6, 27 and 28 and contains the amino acid sequence corresponding to Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  and that reversibly binds to the agent; or 
 c) a functional fragment of a) or b) that reversibly binds to the agent. 
 
     
     
         50 . The cell surface conjugate of  claim 48  or  claim 49 , wherein the streptavidin analog or mutein further comprises an amino acid replacement or replacements at a position corresponding to 117, 120 and/or 121 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         51 . The cell surface conjugate of  claim 50 , wherein:
 the amino acid replacement or replacements are selected from among Glu 117 , Asp 117 , Arg 117 , Ser 120 , Ala 120 , Gly 120 , Trp 121 , Tyr 121  or Phe 121 ; or   the amino acid replacement or replacements are selected from one or more of Glu 117 , Gly 120  or Tyr 121 ; or   the amino acid replacements are selected from Glu 117 , Gly 120  or Tyr 121 .   
     
     
         52 . The cell surface conjugate of any of claims any of  claims 1 ,  2 ,  5 ,  7 - 28  and  34 - 51 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in SEQ ID NO: 27 or 28; 
 b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to SEQ ID NOS: 27 or 28 and contains the amino acid sequence corresponding to Val 44 , Thr 45 , Ala 46 , Arg 47 , Glu 117 , Gly 120  and Tyr 121  and reversibly binds to the agent; or 
 c) a functional fragment of a) or b) that reversibly binds to the agent. 
 
     
     
         53 . The cell surface conjugate of any of  claims 3 - 5 ,  17 - 19  and  35 - 37 , wherein the competition substance is or comprises biotin, a biotin analog or a biologically active fragment thereof. 
     
     
         54 . The cell surface conjugate of any of  claims 1 - 53 , wherein the agent is an affinity tag. 
     
     
         55 . The cell surface conjugate of any of  claims 3 ,  4 ,  6 ,  8 - 10 ,  17 - 19 ,  21 - 27 ,  29 - 33 ,  35 - 37 ,  39 - 47  and  54 , wherein the agent is or comprises a Strep tag, His tag, Flag tag, Xpress tag, Avi tag, Calmodulin tag, Polyglutamate tag, HA tag, Myc tag, Nus tag, S tag, X tag, SBP tag, Softag, V5 tag, CBP, GST, MBP, GFP, Thioredoxin tag, or any combination thereof. 
     
     
         56 . The cell surface conjugate of any of  claims 1 - 55 , wherein the agent is or comprises one or more streptavidin binding peptide, which optionally is a Strep tag. 
     
     
         57 . The cell surface conjugate of  claim 56 , wherein the streptavidin binding peptide comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 8) or Trp-Arg-His-Pro-Gln-Phe-Gly-Gly (SEQ ID NO:7). 
     
     
         58 . The cell surface conjugate of  claim 56  or  claim 57 , wherein the agent comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 3 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 17), Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 18) and Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 Gly-Gly-Ser-Ala-Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 19). 
     
     
         59 . The cell surface conjugate of any of  claims 9 - 58 , wherein the reference cell surface molecule is a cell surface receptor, ligand, glycoprotein, cell adhesion molecule, antigen, integrin or cluster of differentiation (CD). 
     
     
         60 . The cell surface conjugate of  claim 59 , wherein the reference cell surface molecule is a cell surface receptor. 
     
     
         61 . The cell surface conjugate of any of  claims 9 - 60 , wherein the reference cell surface molecule is selected from EpCAM, VEGFR, integrin, optionally integrins αvβ3, α4, αIIbβ3, α4β7, α5β1, αvβ3 or αv, a member of the TNF receptor superfamily, optionally TRAIL-R1 or TRAIL-R2, a member of the epidermal growth factor receptor family, PDGF Receptor, interferon receptor, folate receptor, GPNMB, ICAM-1, HLA-DR, CEA, CA-125, MUC1, TAG-72, IL-6 receptor, 5T4, GD2, GD3, prostate-specific membrane antigen (PSMA) or a clusters of differentiation cell surface molecule, optionally CD2, CD3, CD4, CD5, CD 11, CD11a/LFA-1, CD15, CD18/ITGB2, CD19, CD20, CD22, CD23/IgE Receptor, CD25, CD28, CD30, CD33, CD38, CD40, CD41, CD44, CD51, CD52, CD62L, CD74, CD80, CD125, CD147/basigin, CD152/CTLA-4, CD154/CD40L, CD195/CCR5 and CD319/SLAMF7. 
     
     
         62 . The cell surface conjugate of any of  claims 9 - 61 , wherein the reference cell surface molecule is a member of the epidermal growth factor receptor family. 
     
     
         63 . The cell surface conjugate of any of  claims 9 - 62 , wherein the reference cell surface molecule is an epidermal growth factor receptor (EGFR), an erbB-2 receptor tyrosine-protein kinase (errb2, HER2), an erbB-3 receptor tyrosine-protein kinase, an erbB-4 receptor tyrosine-protein kinase, a hepatocyte growth factor receptor (HGFR/c-MET) or an insulin-like growth factor receptor-1 (IGF-1 R). 
     
     
         64 . The cell surface conjugate of any of  claims 9 - 63 , wherein the reference cell surface molecule is human. 
     
     
         65 . The cell surface conjugate of any of  claims 9 - 64 , wherein the modified cell surface molecule lacks a functional intracellular signaling domain and/or is not capable of mediating intracellular signaling. 
     
     
         66 . The cell surface conjugate of any of  claims 9 - 65 , wherein the modified cell surface molecule is truncated to lack all or a portion of the intracellular signaling domain or trafficking domain compared to the reference cell surface molecule. 
     
     
         67 . The cell surface conjugate of any of  claims 9 - 66 , wherein the modified cell surface molecule exhibits altered cellular internalization, enzymatic activity and/or ligand binding, compared to the reference cell surface molecule. 
     
     
         68 . The cell surface conjugate of any of  claims 9 - 67 , wherein the modified cell surface molecule comprises one or more extracellular domains of the reference cell surface molecule. 
     
     
         69 . The cell surface conjugate of any of  claims 9 - 68 , wherein the modified cell surface molecule is capable of binding to a native ligand and/or substrate of the reference cell surface molecule. 
     
     
         70 . The cell surface conjugate of any of  claims 9 - 68 , wherein the modified cell surface molecule is reduced for or does not bind the native ligand and/or substrate of the reference cell surface molecule. 
     
     
         71 . The cell surface conjugate of  claim 70 , wherein the modified cell surface molecule comprises at least one extracellular domain of the reference cell surface molecule but lacks one or more other extracellular domains recognized by the native ligand and/or substrate of the reference cell surface molecule. 
     
     
         72 . The cell surface conjugate of  claim 71 , wherein the at least one extracellular domain comprises an epitope recognized by an antibody or antigen-binding fragment thereof that specifically binds the reference cell surface molecule. 
     
     
         73 . The cell surface conjugate of any of  claims 46 - 72 , wherein the antibody or antigen-binding fragment is selected from AMG-102, AMG-479, BIIB022OA-5D5, CP-751,871, IMC-A12, R1507, 3F8, abagovomab, abciximab, adecatumumab, afutuzumab, alemtuzumab, altumomab pentetate, anatumomab mafenatox, apolizumab, arcitumomab, aselizumab, atlizumab (=tocilizumab), basiliximab, bectumomab, benralizumab, besilesomab, bivatuzumab mertansine, blinatumomab, brentuximab vedotin, cantuzumab mertansine, capromab pendetide, catumaxomab, CC49, cedelizumab, celmoleukin, cetuximab, cixutumumab, clenoliximab, clivatuzumab tetraxetan, CNTO-95, conatumumab, dacetuzumab, daclizumab, daratumumab, detumomab, ecromeximab, ertumaxomab, edrecolomab, efalizumab, elotuzumab, enlimomab pegol, epitumomab cituxetan, epratuzumab, erlizumab, etaracizumab, fanolesomab, faralimomab, farletuzumab, figitumumab, galiximab, gavilimomab, gemtuzumab ozogamicin, glembatumumab vedotin, gomiliximab, ibalizumab, ibritumomab tiuxetan, igovomab, intetumumab, iratumumab, inolimomab, inotuzumab ozogamicin, ipilimumab, keliximab, labetuzumab, lintuzumab, lexatumumab, lucatumumab, lumiliximab, mapatumumab, maslimomab, matuzumab, milatuzumab, minretumomab, mitumomab, muromonab-CD3, naptumomab estafenatox, natalizumab, necitumumab, ocrelizumab, odulimomab, ofatumumab, olaratumab, oportuzumab monatox, oregovomab, otelixizumab, panitumumab, pertuzumab, pemtumomab, priliximab, PRO 140, nimotuzumab, robatumumab, rituximab, rovelizumab, ruplizumab, satumomab pendetide, siplizumab, sontuzumab, tadocizumab, taplitumomab paptox, teneliximab, teplizumab, TGN1412, ticilimumab (=tremelimumab), tigatuzumab, tocilizumab (=atlizumab), toralizumab, tositumomab, trastuzumab, tremelimumab, tucotuzumab, vedolizumab, veltuzumab, visilizumab, vitaxin, volociximab, votumumab, zalutumumab, zanolimumab, ziralimumab, zolimomab aritox, Atezolizumab, bevacizumab (Avastin @), denosumab, dinutuximab, nivolumab, obinutuzumab, pembrolizumab, pidilizumab (CT-011), ramucirumab, siltuximab, ado-trastuzumab emtansine, CEA-scan Fab fragment, OC125 monoclonal antibody, ab75705, B72.3, MPDL3280A, MSB001078C, MEDI4736, or an antigen binding fragment thereof. 
     
     
         74 . The cell surface conjugate of any of  claims 9 - 73 , wherein the reference cell surface molecule is a reference EGFR and the modified cell surface molecule is a modified EGFR. 
     
     
         75 . The cell surface conjugate of  claim 74  wherein the modified EGFR comprises an epitope specifically recognized by cetuximab or an antigen binding fragment thereof. 
     
     
         76 . The cell surface conjugate of  claim 74  or  claim 75 , wherein the modified EGFR lacks one or more of an EGFR Domain I, an EGFR Domain II, an EGFR Juxtamembrane Domain, and an EGFR Tyrosine Kinase Domain of the reference EGFR. 
     
     
         77 . The cell surface conjugate of any of  claims 74 - 76 , wherein the modified EGFR lacks all of the domains EGFR Domain I, an EGFR Domain II, an EGFR Juxtamembrane Domain, and an EGFR Tyrosine Kinase Domain of the reference EGFR. 
     
     
         78 . The cell surface conjugate of any of  claims 74 - 77 , wherein the modified EGFR comprises an extracellular domain that consists of or consists essentially of subdomain III and subdomain IV of the reference EGFR. 
     
     
         79 . The cell surface conjugate of any of  claims 74 - 78 , wherein the modified EGFR comprises the sequence of amino acids set forth in SEQ ID NOS: 44 or 46 or a sequence of amino acids that exhibits at least at or about 85%, 90%, or 95% sequence identity to SEQ ID NOS: 44 or 46. 
     
     
         80 . The cell surface conjugate of any of claim  973 , wherein the reference cell surface molecule is a reference HER2 and the modified cell surface molecule is a modified HER2. 
     
     
         81 . The cell surface conjugate of  claim 80 , wherein the modified HER2 comprises an epitope specifically recognized by trastuzumab or an antigen binding fragment thereof. 
     
     
         82 . The cell surface conjugate of  claim 80  or  claim 81 , wherein the modified HER2 lacks one or more of an HER2 Domain I, an HER2 Domain II, an HER2 Domain III of the reference HER2. 
     
     
         83 . The cell surface conjugate of any of  claims 80 - 82 , wherein the modified HER2 lacks all of the domains HER2 Domain I, HER2 Domain II, and HER2 Domain III of the reference EGFR of the reference HER2. 
     
     
         84 . The cell surface conjugate of any of  claims 80 - 83 , wherein the modified HER2 comprises an extracellular domain that consists of or consists essentially of Domain IV of the reference HER2. 
     
     
         85 . The cell surface conjugate of any of  claims 80 - 84 , wherein the modified HER2 comprises the sequence of amino acids set forth in SEQ ID NO: 92 or a sequence of amino acids that exhibits at least at or about 85%, 90%, or 95% sequence identity to SEQ ID NO: 92. 
     
     
         86 . The cell surface conjugate of any of  claims 9 - 72 , wherein the reference cell surface molecule is a reference PSMA and the modified cell surface molecule is a modified PSMA. 
     
     
         87 . The cell surface conjugate of  claim 86 , wherein the reference PSMA is a wild-type PSMA, optionally wild-type human PSMA. 
     
     
         88 . The cell surface conjugate of  claim 87 , wherein the reference PSMA is a human PSMA and/or comprises the sequence of amino acids set forth in SEQ ID NO: 94 or a sequence of amino acids encoded by the sequence of nucleotides set forth in SEQ ID NO: 96 or 97. 
     
     
         89 . The cell surface conjugate of any of  claims 86 - 88 , wherein the modified PSMA comprises an extracellular portion and a transmembrane domain of the reference PSMA. 
     
     
         90 . The cell surface conjugate of any of  claims 86 - 89 , wherein the modified PSMA comprises one or more amino acid modifications in the intracellular region compared to the reference PSMA. 
     
     
         91 . The cell surface conjugate of any of  claims 86 - 90 , wherein the one or more amino acid modification comprises one or more amino acid substitutions, deletions and/or insertions. 
     
     
         92 . The cell surface conjugates of any of  claims 86 - 91 , wherein the modified PSMA exhibits altered cellular internalization compared to the reference PSMA. 
     
     
         93 . The cell surface conjugate of any of  claims 86 - 92 , wherein the modified PSMA comprises an amino acid substitution corresponding to W2G or does not comprise W2 or does not comprise any residue at position 2, with reference to positions in the sequence of amino acids set forth in SEQ ID NO:94. 
     
     
         94 . The cell surface conjugate of any of  claims 86 - 93 , wherein the modified PSMA comprises a deletion or truncation of 11 N-terminal amino acids, compared to the reference PSMA. 
     
     
         95 . The cell surface conjugate of any of  claims 86 - 94 , wherein the modified PSMA comprises an epitope capable of being recognized by an antibody or antigen-binding fragment thereof. 
     
     
         96 . The cell surface conjugate of  claim 95 , wherein the antibody or antigen-binding fragment thereof is selected from among J591, DFO-J591, CYT-356, J415, 3/A12, 3/F11, 3/E7, D2B, 107-1A4, YPSMA-1, YPSMA-2, 3E6, 2G7, 24.4E6, GCP-02, GCP-04, GCP-05, J533, E99, 1G9, 3C6, 4.40, 026, D7-Fc, D7-CH3, 4D4, A5, and antigen-binding fragments thereof. 
     
     
         97 . The cell surface conjugate of any of  claims 1 - 96 , wherein the cell surface conjugate is not immunogenic and/or does not induce an immune response in a subject in which it is administered. 
     
     
         98 . A polynucleotide, comprising a nucleic acid sequence encoding the cell surface conjugate of any of  claims 1 - 97 . 
     
     
         99 . The polynucleotide of  claim 98 , wherein the nucleic acid sequence further comprising a signal sequence. 
     
     
         100 . The polynucleotide of  claim 99 , wherein the signal sequence encodes a signal peptide derived from GMCSFR alpha chain. 
     
     
         101 . The polynucleotide of any of  claims 98 - 100 , wherein the nucleic acid sequence is a first nucleic acid sequence and the polynucleotide further comprises a second nucleic acid sequence encoding a recombinant receptor. 
     
     
         102 . The polynucleotide of  claim 101 , wherein the recombinant receptor is or comprises a chimeric antigen receptor (CAR). 
     
     
         103 . The polynucleotide of  claim 101  or  claim 102 , wherein the first and second nucleic acid sequences are separated by an internal ribosome entry site (IRES), or a nucleotide sequence encoding a self-cleaving peptide or a peptide that causes ribosome skipping, which optionally is a T2A, a P2A, an E2A or an F2A. 
     
     
         104 . The polynucleotide of any of  claims 101 - 103 , wherein the first nucleic acid sequence is upstream of the second nucleic acid sequence. 
     
     
         105 . The polynucleotide of any of  claims 101 - 103 , wherein the first nucleic acid sequence is downstream of the second nucleic acid sequence. 
     
     
         106 . A vector, comprising the polynucleotide of any of  claims 98 - 105 . 
     
     
         107 . The vector of  claim 106  that is a viral vector. 
     
     
         108 . The vector of  claim 106  or  claim 107  that is a retroviral vector. 
     
     
         109 . The vector of any of  claims 106 - 108  that is a lentiviral vector or a gammaretroviral vector. 
     
     
         110 . A method of producing an engineered cell, comprising introducing the polynucleotide of any of  claims 96 - 105  or the vector of any of  claims 106 - 109  into a cell. 
     
     
         111 . An engineered cell produced by the method of  claim 110 . 
     
     
         112 . An engineered cell, comprising the polynucleotide of any of  claims 98 - 105  or the vector of any of  claims 106 - 109 . 
     
     
         113 . An engineered cell, comprising the cell surface conjugate of any of  claims 1 - 97 . 
     
     
         114 . The engineered cell of  claim 113 , further comprising a recombinant receptor. 
     
     
         115 . The engineered cell of  claim 114 , wherein the recombinant receptor is capable of binding to a target antigen that is associated with, specific to, and/or expressed on a cell or tissue of a disease or disorder. 
     
     
         116 . The engineered cell of  claim 115 , wherein the disease or disorder is an infectious disease or disorder, an autoimmune disease, an inflammatory disease, or a tumor or a cancer. 
     
     
         117 . The engineered cell of  claim 115  or  claim 116 , wherein the target antigen is a tumor antigen. 
     
     
         118 . The engineered cell of any of  claims 115 - 117 , wherein the target antigen is selected from the group consisting of αvβ6 integrin (avb6 integrin), B cell maturation antigen (BCMA), B7-H3, B7-H6, carbonic anhydrase 9 (CA9, also known as CAIX or G250), a cancer-testis antigen, cancer/testis antigen 1B (CTAG, also known as NY-ESO-1 and LAGE-2), carcinoembryonic antigen (CEA), a cyclin, cyclin A2, C-C Motif Chemokine Ligand 1 (CCL-1), CD19, CD20, CD22, CD23, CD24, CD30, CD33, CD38, CD44, CD44v6, CD44v7/8, CD123, CD138, CD171, epidermal growth factor protein (EGFR), truncated epidermal growth factor protein (tEGFR), type III epidermal growth factor receptor mutation (EGFR vIII), epithelial glycoprotein 2 (EPG-2), epithelial glycoprotein 40 (EPG-40), ephrinB2, ephhrine receptor A2 (EPHa2), estrogen receptor, Fc receptor like 5 (FCRL5; also known as Fc receptor homolog 5 or FCRH5), fetal acetylcholine receptor (fetal AchR), a folate binding protein (FBP), folate receptor alpha, ganglioside GD2, 0-acetylated GD2 (OGD2), ganglioside GD3, glycoprotein 100 (gp100), G Protein Coupled Receptor 5D (GPCR5D), Her2/neu (receptor tyrosine kinase erb-B2), Her3 (erb-B3), Her4 (erb-B4), erbB dimers, Human high molecular weight-melanoma-associated antigen (HMW-MAA), hepatitis B surface antigen, Human leukocyte antigen A1 (HLA-A1), Human leukocyte antigen A2 (HLA-A2), IL-22 receptor alpha(IL-22Ra), IL-13 receptor alpha 2 (IL-13Ra2), kinase insert domain receptor (kdr), kappa light chain, L1 cell adhesion molecule (L1-CAM), CE7 epitope of L1-CAM, Leucine Rich Repeat Containing 8 Family Member A (LRRC8A), Lewis Y, Melanoma-associated antigen (MAGE)-A1, MAGE-A3, MAGE-A6, mesothelin, c-Met, murine cytomegalovirus (CMV), mucin 1 (MUC1), MUC16, natural killer group 2 member D (NKG2D) ligands, melan A (MART-1), neural cell adhesion molecule (NCAM), oncofetal antigen, Preferentially expressed antigen of melanoma (PRAME), progesterone receptor, a prostate specific antigen, prostate stem cell antigen (PSCA), prostate specific membrane antigen (PSMA), Receptor Tyrosine Kinase Like Orphan Receptor 1 (ROR1), survivin, Trophoblast glycoprotein (TPBG also known as 5T4), tumor-associated glycoprotein 72 (TAG72), vascular endothelial growth factor receptor (VEGFR), vascular endothelial growth factor receptor 2 (VEGFR2), Wilms Tumor 1 (WT-1) 
     
     
         119 . The engineered cell of any of  claims 115 - 118 , wherein the target antigen is selected from the group consisting of ROR1, HER2, L1-CAM, CD19, CD20, CD22, mesothelin, CEA, hepatitis B surface antigen, anti-folate receptor, CD23, CD24, CD30, CD33, CD38, CD44, EGFR, EGP-2, EGP-4, EPHa2, ErbB2, ErbB3, ErbB4, FBP, fetal acethycholine e receptor, GD2, GD3, HMW-MAA, IL-22R-alpha, IL-13R-alpha2, kdr, kappa light chain, Lewis Y, L1-cell adhesion molecule, MAGE-A1, mesothelin, MUC1, MUC16, PSCA, NKG2D Ligands, NY-ESO-1, MART-1, gp100, oncofetal antigen, TAG72, VEGF-R2, carcinoembryonic antigen (CEA), prostate specific antigen, PSMA, estrogen receptor, progesterone receptor, ephrinB2, CD123, CS-1, c-Met, GD-2, MAGE A3, CE7, Wilms Tumor 1 (WT-1), and cyclin A1 (CCNA1). 
     
     
         120 . The engineered cell of any of  claims 114 - 119 , wherein the recombinant receptor is a functional non-TCR antigen receptor or a transgenic TCR. 
     
     
         121 . The engineered cell of any of  claims 114 - 120 , wherein the recombinant receptor is a chimeric antigen receptor (CAR). 
     
     
         122 . The engineered cell of any of  claims 114 - 121 , wherein the recombinant receptor comprises an extracellular portion comprising an antigen-binding domain. 
     
     
         123 . The engineered cell of  claim 122 , wherein the antigen-binding domain is or comprises an antibody or an antibody fragment. 
     
     
         124 . The engineered cell of  claim 123 , wherein the antibody fragment is a single chain fragment. 
     
     
         125 . The engineered cell of  claim 123  or  claim 124 , wherein the fragment comprises antibody variable regions joined by a flexible linker. 
     
     
         126 . The engineered cell of any of  claims 123 - 125 , wherein the fragment comprises an scFv. 
     
     
         127 . The engineered cell of any of  claims 114 - 126 , wherein the recombinant receptor comprises an intracellular signaling region. 
     
     
         128 . The engineered cell of  claim 127 , wherein the intracellular signaling region comprises an intracellular signaling domain. 
     
     
         129 . The engineered cell of  claim 128 , wherein the intracellular signaling domain is or comprises a primary signaling domain, a signaling domain that is capable of inducing a primary activation signal in a T cell, a signaling domain of a T cell receptor (TCR) component, and/or a signaling domain comprising an immunoreceptor tyrosine-based activation motif (ITAM). 
     
     
         130 . The engineered cell of  claim 128  or  claim 129 , wherein the intracellular signaling domain is or comprises an intracellular signaling domain of a CD3 chain, optionally a CD3-zeta (CD3ζ) chain or a signaling portion thereof. 
     
     
         131 . The engineered cell of any of  claims 127 - 130 , further comprising a transmembrane domain disposed between the extracellular domain and the intracellular signaling region. 
     
     
         132 . The engineered cell of any of  claims 127 - 131 , wherein the intracellular signaling region further comprises a costimulatory signaling domain. 
     
     
         133 . The engineered cell of  claim 132 , wherein the costimulatory signaling domain comprises an intracellular signaling domain of a T cell costimulatory molecule or a signaling portion thereof. 
     
     
         134 . The engineered cell of  claim 132  or  claim 133 , wherein the costimulatory signaling domain comprises an intracellular signaling domain of a CD28, a 4-1BB or an ICOS or a signaling portion thereof. 
     
     
         135 . The engineered cell of any of  claims 132 - 134 , wherein the costimulatory signaling domain is between the transmembrane domain and the intracellular signaling domain. 
     
     
         136 . The engineered cell of any of  claims 111 - 135 , wherein the cell is an immune cell. 
     
     
         137 . The engineered cell of  claim 136 , wherein the cell is a lymphocyte. 
     
     
         138 . The engineered cell of any of  claims 111 - 137 , wherein the cell is a T cell or an NK cell. 
     
     
         139 . The engineered cell of  claim 138 , wherein the cell is a T cell that is a CD8+ T cell or a CD4+ T cell. 
     
     
         140 . A composition comprising the engineered cells of any of  claims 111 - 139 . 
     
     
         141 . The composition of  claim 140 , further comprising a pharmaceutically acceptable excipient. 
     
     
         142 . A method of treatment comprising administering the engineered cells of any of  claims 111 - 139  or the composition of  claim 140  or  claim 141  to a subject having a disease or disorder. 
     
     
         143 . The method of  claim 142 , wherein the disease or disorder is a cancer, a tumor, an autoimmune disease or disorder, or an infectious disease. 
     
     
         144 . The method of  claim 142  or  claim 143 , further comprising administering to the subject a binding molecule capable of recognizing the agent of the cell surface conjugate expressed on the engineered cell and detecting cells that express the cell surface conjugate. 
     
     
         145 . The method of  claim 144 , wherein detection comprises in vivo imaging. 
     
     
         146 .- 151 . (canceled) 
     
     
         152 . A method of identifying cells transduced with a cell surface conjugate, comprising:
 (a) contacting a composition transduced with a polynucleotide of any of  claims 98 - 105  or the vector of any of  claims 106 - 109  encoding the cell surface conjugate or the engineered cell of any of  claims 111 - 139  or the composition of  claim 140  or  claim 141  with a binding molecule capable of recognizing the agent of the cell surface conjugate; and   (b) identifying cells bound to the binding molecule.   
     
     
         153 . A method of identifying cells transduced with a cell surface conjugate, comprising:
 (a) introducing a polynucleotide of any of  claims 98 - 105  or the vector of any of  claims 106 - 109  encoding the cell surface conjugate into a cell;   (b) contacting a composition comprising the cell of (a) with a binding molecule capable of recognizing the agent of the cell surface conjugate; and   (c) identifying cells of the composition bound to the binding molecule.   
     
     
         154 . A method of selecting cells transduced with a cell surface conjugate, comprising:
 (a) contacting a composition transduced with a polynucleotide of any of  claims 98 - 105  or the vector of any of  claims 106 - 109  encoding the cell surface conjugate or the engineered cell of any of  claims 111 - 139  or the composition of  claim 140  or  claim 141  with a binding molecule capable of recognizing the agent of the cell surface conjugate; and   (b) isolating cells bound to the binding molecule.   
     
     
         155 . A method of selecting cells transduced with a cell surface conjugate, comprising:
 (a) introducing a polynucleotide of any of  claims 98 - 105  or the vector of any of  claims 106 - 109  encoding the cell surface conjugate into a cell;   (b) contacting a composition comprising the cell of (a) with a binding molecule capable of recognizing the agent of the cell surface conjugate; and   (c) isolating cells of the composition bound to the binding molecule.   
     
     
         156 . The method of  claim 154  or  claim 155 , wherein the binding molecule is conjugated to a detectable moiety or is capable of producing a detectable signal. 
     
     
         157 . The method of  claim 156 , wherein the detectable moiety comprises a fluorescent protein. 
     
     
         158 . The method of any of  claims 144 ,  145 , or  152 - 157 , wherein the agent is a streptavidin binding peptide. 
     
     
         159 . The method of  claim 158 , wherein the streptavidin binding peptide is or comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 8) or Trp-Arg-His-Pro-Gln-Phe-Gly-Gly (SEQ ID NO:7). 
     
     
         160 . The method of  claim 159 , wherein the streptavidin binding peptide is or comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 3 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 17), Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 18) and Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 Gly-Gly-Ser-Ala-Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 19). 
     
     
         161 . The method of any of  claims 144 ,  145 , or  152 - 160 , wherein the binding molecule is a reagent capable of reversibly binding to the agent and/or capable of being competed in the presence of a competition substance. 
     
     
         162 . The method of  claim 161 , wherein the reagent is a streptavidin, a streptavidin analog or mutein. 
     
     
         163 . The method of  claim 162 , wherein the streptavidin analog or mutein comprises the amino acid sequence Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  at sequence positions corresponding to positions 44 to 47 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         164 . The method of  claim 162  or  claim 163 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in any of SEQ ID NOS: 3-6, 27 and 28; 
 b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to any of SEQ ID NOS: 3-6, 27 and 28 and contains the amino acid sequence corresponding to Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  and that reversibly binds to the agent; or 
 c) a functional fragment of a) or b) that reversibly binds to the agent. 
 
     
     
         165 . The method of  claim 163  or  claim 164 , wherein the streptavidin analog or mutein further comprises an amino acid replacement or replacements at a position corresponding to 117, 120 and/or 121 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         166 . The method of  claim 165 , wherein:
 the amino acid replacement or replacements are selected from among Glu 117 , Asp 117 , Arg 117 , Ser 120 , Ala 120 , Gly 120 , Trp 121 , Tyr 121  or Phe 121 ; or   the amino acid replacement or replacements are selected from one or more of Glu 117 , Gly 120  or Tyr 121 ; or   the amino acid replacements are selected from Glu 117 , Gly 120  or Tyr 120 .   
     
     
         167 . The method of any of  claims 162 - 166 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in SEQ ID NO: 27 or 28;   b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to SEQ ID NOS:27 or 28 and contains the amino acid sequence corresponding to Val 44 , Thr 45 , Ala 46 , Arg 47 , Glu 117 , Gly 120  and Tyr 121  and reversibly binds to the agent; or   c) a functional fragment of a) or b) that reversibly binds to the agent.   
     
     
         168 . The method of any of  claims 161 - 167 , further comprising disrupting the reversible binding of the binding molecule to the agent. 
     
     
         169 . The method of  claim 168 , wherein said disruption comprises contacting the cells with a composition comprising a competition substance capable of reversing the bond between the binding molecule and agent. 
     
     
         170 . The method of  claim 169 , wherein the competition substance is a free binding partner and/or is a competition agent. 
     
     
         171 . The method of  claim 169  or  claim 170 , wherein the competition substance is or comprises biotin, a biotin analog or a biologically active fragment thereof. 
     
     
         172 . The method of any of  claims 144 ,  145 , or  152 - 171 , wherein the binding molecule is an antibody or antigen binding fragment that specifically binds the agent. 
     
     
         173 . The method of  claim 172 , wherein the binding molecule is an anti-StrepTag antibody. 
     
     
         174 . A molecule, comprising a streptavidin or a streptavidin analog or mutein conjugated to a cytotoxic agent. 
     
     
         175 . The molecule of  claim 174 , comprising a streptavidin analog or mutein. 
     
     
         176 . The molecule of  claim 174  or  claim 175 , wherein the streptavidin or streptavidin analog or mutein binds to a streptavidin binding peptide. 
     
     
         177 . The molecule of  claim 176 , wherein the streptavidin binding peptide is or comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 8) or Trp-Arg-His-Pro-Gln-Phe-Gly-Gly (SEQ ID NO:7). 
     
     
         178 . The molecule of  claim 176  or  claim 177 , wherein the streptavidin binding peptide is or comprises the sequence Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 3 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys ((SEQ ID NO: 17), Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 -Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 18) and Trp-Ser-His-Pro-Gln-Phe-Glu-Lys-(GlyGlyGlySer) 2 Gly-Gly-Ser-Ala-Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (SEQ ID NO: 19). 
     
     
         179 . The molecule of any of  claims 176 - 178 , wherein the streptavidin or streptavidin analog or mutein exhibits a binding affinity for the streptavidin binding peptide with an equilibrium dissociation constant (K D ) of from or from about 10 −4  M to or to about 10 −10  M. 
     
     
         180 . The molecule of any of  claims 174 - 179 , wherein the streptavidin analog or mutein comprises the amino acid sequence Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  at sequence positions corresponding to positions 44 to 47 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         181 . The molecule of any of  claims 174 - 180 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in any of SEQ ID NOS: 3-6, 27 and 28;   b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to any of SEQ ID NOS: 3-6, 27 and 28 and contains the amino acid sequence corresponding to Val 44 -Thr 45 -Ala 46 -Arg 47  or Ile 44 -Gly 45 -Ala 46 -Arg 47  and that reversibly binds to the agent; or   c) a functional fragment of a) or b) that binds to the streptavidin binding peptide.   
     
     
         182 . The molecule of  claim 180  or  claim 181 , wherein the streptavidin analog or mutein further comprises an amino acid replacement or replacements at a position corresponding to 117, 120 and/or 121 with reference to positions in streptavidin in the sequence of amino acids set forth in SEQ ID NO:1. 
     
     
         183 . The molecule of  claim 182 , wherein:
 the amino acid replacement or replacements are selected from among Glu 117 , Asp 117 , Arg 117 , Ser 120 , Ala 120 , Gly 120 , Trp 121 , Tyr 121  or Phe 12 L; or   the amino acid replacement or replacements are selected from one or more of Glu 117 , Gly 120  or Tyr 121 ; or   the amino acid replacements are selected from Glu 117 , Gly 120  or Tyr 121 .   
     
     
         184 . The molecule of any of  claims 174 - 183 , wherein the streptavidin analog or mutein comprises:
 a) the sequence of amino acids set forth in SEQ ID NO: 27 or 28;   b) a sequence of amino acids that exhibits at least at or about 85%, 86%, 87%, 88%, 89%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to SEQ ID NOS:27 or 28 and contains the amino acid sequence corresponding to Val 44 , Thr 45 , Ala 46 , Arg 47 , Glu 117 , Gly 120  and Tyr 121  and reversibly binds to the agent; or   c) a functional fragment of a) or b) that reversibly binds to the streptavidin binding peptide.   
     
     
         185 . The molecule of any of  claims 174 - 184 , wherein the cytotoxic agent is a toxin. 
     
     
         186 . The molecule of  claim 185 , wherein the toxin is a peptide toxin, ricin A chain toxin, Abrin A chain, Diptheria Toxin (DT) A chain,  Pseudomonas  exotoxin, Shiga Toxin A chain, Gelonin, Momordin, Pokeweed Antiviral Protein, Saporin, Trichosanthin, or Barley Toxin. 
     
     
         187 . The molecule of  claim 185 , wherein the toxin is a phototoxin. 
     
     
         188 . A method of killing cells, comprising administering the molecule of any of  claims 174 - 187  to a subject previously administered the cells of any of  claims 111 - 139  or the composition of  claim 140  or  claim 141 . 
     
     
         189 . The method of  claim 188 , wherein the molecule is administered at a time at which the subject is exhibiting a toxic outcome associated with the administered cells or at a time at which the subject is exhibiting a detectable and/or cell-mediated immune response to the administered cells. 
     
     
         190 . The method of  claim 189 , wherein the toxic outcome is associated with neurotoxicity or cytokine release syndrome (CRS).