Michael S Kuhns
- Assistant Professor, Immunobiology
- Assistant Professor, BIO5 Institute
- Assistant Professor, Genetics - GIDP
- Ph.D. Molecular and Cellular Biology
- University of California at Berkeley, Berkeley
- CTLA-4-mediated tuning of secondary CD4+ T cell responses.
- B.S. Biochemistry
- University of Arizona, Tucson, Arizona, United States
- Magna Cum Laude
- Pew Scholar in the Biomedical Sciences
- Pew Charitable Trusts, Spring 2011
No activities entered.
Basic and Advanced ImmunologyIMB 548 (Fall 2016)
Honors ThesisBIOC 498H (Fall 2016)
Directed ResearchBIOC 492 (Summer I 2016)
CBIO GIDP Seminar SeriesCBIO 596H (Spring 2016)
DissertationIMB 920 (Spring 2016)
Basic and Advanced ImmunologyIMB 548 (Fall 2015)
DissertationIMB 920 (Fall 2015)
ResearchIMB 900 (Fall 2015)
DissertationIMB 920 (Spring 2015)
ResearchIMB 900 (Spring 2015)
Research SeminarIMB 696A (Spring 2015)
Basic and Advanced ImmunologyIMB 548 (Fall 2014)
DissertationIMB 920 (Fall 2014)
Introduction to ResearchMCB 795A (Fall 2014)
ResearchIMB 900 (Fall 2014)
Research SeminarIMB 696A (Fall 2014)
Directed RsrchMCB 492 (Spring 2014)
IMB- Journal ClubIMB 595A (Spring 2014)
ResearchIMB 900 (Spring 2014)
Research SeminarIMB 696A (Spring 2014)
Basic and Advanced ImmunologyIMB 548 (Fall 2013)
Directed RsrchMCB 492 (Fall 2013)
IMB- Journal ClubIMB 595A (Fall 2013)
ResearchIMB 900 (Fall 2013)
Research SeminarIMB 696A (Fall 2013)
- Bethune, M. T., Gee, M. H., Bunse, M., Lee, M. S., Gschweng, E. H., Pagadala, M. S., Zhou, J., Cheng, D., Heath, J. R., Kohn, D. B., Kuhns, M. S., Uckert, W., & Baltimore, D. (2016). Domain-swapped T cell receptors improve the safety of TCR gene therapy. eLife, 5.More infoT cells engineered to express a tumor-specific αβ T cell receptor (TCR) mediate anti-tumor immunity. However, mispairing of the therapeutic αβ chains with endogenous αβ chains reduces therapeutic TCR surface expression and generates self-reactive TCRs. We report a general strategy to prevent TCR mispairing: swapping constant domains between the α and β chains of a therapeutic TCR. When paired, domain-swapped (ds)TCRs assemble with CD3, express on the cell surface, and mediate antigen-specific T cell responses. By contrast, dsTCR chains mispaired with endogenous chains cannot properly assemble with CD3 or signal, preventing autoimmunity. We validate this approach in cell-based assays and in a mouse model of TCR gene transfer-induced graft-versus-host disease. We also validate a related approach whereby replacement of αβ TCR domains with corresponding γδ TCR domains yields a functional TCR that does not mispair. This work enables the design of safer TCR gene therapies for cancer immunotherapy.
- Glassman, C. R., Parrish, H. L., Deshpande, N. R., & Kuhns, M. S. (2016). The CD4 and CD3δε Cytosolic Juxtamembrane Regions Are Proximal within a Compact TCR-CD3-pMHC-CD4 Macrocomplex. Journal of immunology (Baltimore, Md. : 1950), 196(11), 4713-22.More infoTCRs relay information about peptides embedded within MHC molecules (pMHC) to the ITAMs of the associated CD3γε, CD3δε, and CD3ζζ signaling modules. CD4 then recruits the Src kinase p56(Lck) (Lck) to the TCR-CD3 complex to phosphorylate the ITAMs, initiate intracellular signaling, and drive CD4(+) T cell fate decisions. Whereas the six ITAMs of CD3ζζ are key determinants of T cell development, activation, and the execution of effector functions, multiple models predict that CD4 recruits Lck proximal to the four ITAMs of the CD3 heterodimers. We tested these models by placing FRET probes at the cytosolic juxtamembrane regions of CD4 and the CD3 subunits to evaluate their relationship upon pMHC engagement in mouse cell lines. The data are consistent with a compact assembly in which CD4 is proximal to CD3δε, CD3ζζ resides behind the TCR, and CD3γε is offset from CD3δε. These results advance our understanding of the architecture of the TCR-CD3-pMHC-CD4 macrocomplex and point to regions of high CD4-Lck + ITAM concentrations therein. The findings thus have implications for TCR signaling, as phosphorylation of the CD3 ITAMs by CD4-associated Lck is important for CD4(+) T cell fate decisions.
- Parrish, H. L., Deshpande, N. R., Vasic, J., & Kuhns, M. S. (2016). Functional evidence for TCR-intrinsic specificity for MHCII. Proceedings of the National Academy of Sciences of the United States of America.More infoHow T cells become restricted to binding antigenic peptides within class I or class II major histocompatibility complex molecules (pMHCI or pMHCII, respectively) via clonotypic T-cell receptors (TCRs) remains debated. During development, if TCR-pMHC interactions exceed an affinity threshold, a signal is generated that positively selects the thymocyte to become a mature CD4(+) or CD8(+) T cell that can recognize foreign peptides within MHCII or MHCI, respectively. But whether TCRs possess an intrinsic, subthreshold specificity for MHC that facilitates sampling of the peptides within MHC during positive selection or T-cell activation is undefined. Here we asked if increasing the frequency of lymphocyte-specific protein tyrosine kinase (Lck)-associated CD4 molecules in T-cell hybridomas would allow for the detection of subthreshold TCR-MHC interactions. The reactivity of 10 distinct TCRs was assessed in response to selecting and nonselecting MHCII bearing cognate, null, or "shaved" peptides with alanine substitutions at known TCR contact residues: Three of the TCRs were selected on MHCII and have defined peptide specificity, two were selected on MHCI and have a known pMHC specificity, and five were generated in vitro without defined selecting or cognate pMHC. Our central finding is that IL-2 was made when each TCR interacted with selecting or nonselecting MHCII presenting shaved peptides. These responses were abrogated by anti-CD4 antibodies and mutagenesis of CD4. They were also inhibited by anti-MHC antibodies that block TCR-MHCII interactions. We interpret these data as functional evidence for TCR-intrinsic specificity for MHCII.
- Deshpande, N. R., Parrish, H. L., & Kuhns, M. S. (2015). Self-recognition drives the preferential accumulation of promiscuous CD4(+) T-cells in aged mice. eLife, 4, e05949.More infoT-cell recognition of self and foreign peptide antigens presented in major histocompatibility complex molecules (pMHC) is essential for life-long immunity. How the ability of the CD4(+) T-cell compartment to bind self- and foreign-pMHC changes over the lifespan remains a fundamental aspect of T-cell biology that is largely unexplored. We report that, while old mice (18-22 months) contain fewer CD4(+) T-cells compared with adults (8-12 weeks), those that remain have a higher intrinsic affinity for self-pMHC, as measured by CD5 expression. Old mice also have more cells that bind individual or multiple distinct foreign-pMHCs, and the fold increase in pMHC-binding populations is directly related to their CD5 levels. These data demonstrate that the CD4(+) T-cell compartment preferentially accumulates promiscuous constituents with age as a consequence of higher affinity T-cell receptor interactions with self-pMHC.
- Lee, M. S., Glassman, C. R., Deshpande, N. R., Badgandi, H. B., Parrish, H. L., Uttamapinant, C., Stawski, P. S., Ting, A. Y., & Kuhns, M. S. (2015). A Mechanical Switch Couples T Cell Receptor Triggering to the Cytoplasmic Juxtamembrane Regions of CD3ζζ. Immunity, 43(2), 227-39.More infoThe eight-subunit T cell receptor (TCR)-CD3 complex is the primary determinant for T cell fate decisions. Yet how it relays ligand-specific information across the cell membrane for conversion to chemical signals remains unresolved. We hypothesized that TCR engagement triggers a change in the spatial relationship between the associated CD3ζζ subunits at the junction where they emerge from the membrane into the cytoplasm. Using three in situ proximity assays based on ID-PRIME, FRET, and EPOR activity, we determined that the cytosolic juxtamembrane regions of the CD3ζζ subunits are spread apart upon assembly into the TCR-CD3 complex. TCR engagement then triggered their apposition. This mechanical switch resides upstream of the CD3ζζ intracellular motifs that initiate chemical signaling, as well as the polybasic stretches that regulate signal potentiation. These findings provide a framework from which to examine triggering events for activating immune receptors and other complex molecular machines.
- Parrish, H. L., Glassman, C. R., Keenen, M. M., Deshpande, N. R., Bronnimann, M. P., & Kuhns, M. S. (2015). A Transmembrane Domain GGxxG Motif in CD4 Contributes to Its Lck-Independent Function but Does Not Mediate CD4 Dimerization. PloS one, 10(7), e0132333.More infoCD4 interactions with class II major histocompatibility complex (MHC) molecules are essential for CD4+ T cell development, activation, and effector functions. While its association with p56lck (Lck), a Src kinase, is important for these functions CD4 also has an Lck-independent role in TCR signaling that is incompletely understood. Here, we identify a conserved GGxxG motif in the CD4 transmembrane domain that is related to the previously described GxxxG motifs of other proteins and predicted to form a flat glycine patch in a transmembrane helix. In other proteins, these patches have been reported to mediate dimerization of transmembrane domains. Here we show that introducing bulky side-chains into this patch (GGxxG to GVxxL) impairs the Lck-independent role of CD4 in T cell activation upon TCR engagement of agonist and weak agonist stimulation. However, using Forster's Resonance Energy Transfer (FRET), we saw no evidence that these mutations decreased CD4 dimerization either in the unliganded state or upon engagement of pMHC concomitantly with the TCR. This suggests that the CD4 transmembrane domain is either mediating interactions with an unidentified partner, or mediating some other function such as membrane domain localization that is important for its role in T cell activation.
- Kuhns, M. S., & Badgandi, H. B. (2012). Piecing together the family portrait of TCR-CD3 complexes. Immunological reviews, 250(1), 120-43.More infoThe pre-T-cell receptor (TCR)-, αβTCR-, and γδTCR-CD3 complexes are members of a family of modular biosensors that are responsible for driving T-cell development, activation, and effector functions. They inform essential checkpoint decisions by relaying key information from their ligand-binding modules (TCRs) to their signaling modules (CD3γε + CD3δε and CD3ζζ) and on to the intracellular signaling apparatus. Their actions shape the T-cell repertoire, as well as T-cell-mediated immunity; yet, the mechanisms that underlie their activity remain an enigma. As with any molecular machine, understanding how they function depends upon understanding how their parts fit and work together. In the 30 years since the initial biochemical and genetic characterizations of the αβTCR, the structure and function of the individual components of these family members have been extensively characterized. Cumulatively, this information has allowed us to piece together a portrait of the αβTCR-CD3 complex and outline the form of the remaining family members. Here we review the known structural and functional characteristics of the components of these TCR-CD3 complex family members. We then discuss how these data have informed our understanding of the architecture of the αβTCR-CD3 complex as well as their implications for the other family members. The intent is to provide a framework for considering: (i) how these thematically similar complexes diverge to execute their specific functions and (ii) how our knowledge of the form and function of these distinct family members can cross-inform our understanding of the other family members.
- Kuhns, M. S., & Davis, M. M. (2012). TCR Signaling Emerges from the Sum of Many Parts. Frontiers in immunology, 3, 159.More info"How does T cell receptor signaling begin?" Answering this question requires an understanding of how the parts of the molecular machinery that mediates this process fit and work together. Ultimately this molecular architecture must (i) trigger the relay of information from the TCR-pMHC interface to the signaling substrates of the CD3 molecules and (ii) bring the kinases that modify these substrates in close proximity to interact, initiate, and sustain signaling. In this contribution we will discuss advances of the last decade that have increased our understanding of the complex machinery and interactions that underlie this type of signaling.
- Zeng, X., Wei, Y., Huang, J., Newell, E. W., Yu, H., Kidd, B. A., Kuhns, M. S., Waters, R. W., Davis, M. M., Weaver, C. T., & Chien, Y. (2012). γδ T cells recognize a microbial encoded B cell antigen to initiate a rapid antigen-specific interleukin-17 response. Immunity, 37(3), 524-34.More infoγδ T cells contribute uniquely to immune competence. Nevertheless, how they function remains an enigma. It is unclear what most γδ T cells recognize, what is required for them to mount an immune response, and how the γδ T cell response is integrated into host immune defense. Here, we report that a noted B cell antigen, the algae protein phycoerythrin (PE), is a murine and human γδ T cell antigen. Employing this specificity, we demonstrated that antigen recognition activated naive γδ T cells to make interleukin-17 and respond to cytokine signals that perpetuate the response. High frequencies of antigen-specific γδ T cells in naive animals and their ability to mount effector response without extensive clonal expansion allow γδ T cells to initiate a swift, substantial response. These results underscore the adaptability of lymphocyte antigen receptors and suggest an antigen-driven rapid response in protective immunity prior to the maturation of classical adaptive immunity.
- Newell, E. W., Ely, L. K., Kruse, A. C., Reay, P. A., Rodriguez, S. N., Lin, A. E., Kuhns, M. S., Garcia, K. C., & Davis, M. M. (2011). Structural basis of specificity and cross-reactivity in T cell receptors specific for cytochrome c-I-E(k). Journal of immunology (Baltimore, Md. : 1950), 186(10), 5823-32.More infoT cells specific for the cytochrome c Ag are widely used to investigate many aspects of TCR specificity and interactions with peptide-MHC, but structural information has long been elusive. In this study, we present structures for the well-studied 2B4 TCR, as well as a naturally occurring variant of the 5c.c7 TCR, 226, which is cross-reactive with more than half of possible substitutions at all three TCR-sensitive residues on the peptide Ag. These structures alone and in complex with peptide-MHC ligands allow us to reassess many prior mutagenesis results. In addition, the structure of 226 bound to one peptide variant, p5E, shows major changes in the CDR3 contacts compared with wild-type, yet the TCR V-region contacts with MHC are conserved. These and other data illustrate the ability of TCRs to accommodate large variations in CDR3 structure and peptide contacts within the constraints of highly conserved TCR-MHC interactions.
- Kuhns, M. S., Girvin, A. T., Klein, L. O., Chen, R., Jensen, K. D., Newell, E. W., Huppa, J. B., Lillemeier, B. F., Huse, M., Chien, Y., Garcia, K. C., & Davis, M. M. (2010). Evidence for a functional sidedness to the alphabetaTCR. Proceedings of the National Academy of Sciences of the United States of America, 107(11), 5094-9.More infoThe T cell receptor (TCR) and associated CD3gammaepsilon, deltaepsilon, and zetazeta signaling dimers allow T cells to discriminate between different antigens and respond accordingly, but our knowledge of how these parts fit and work together is incomplete. In this study, we provide additional evidence that the CD3 heterodimers congregate on one side of the TCR in both the alphabeta and gammadeltaTCR-CD3 complexes. We also report that the other side of the alphabetaTCR mediates homotypic alphabetaTCR interactions and signaling. Specifically, an erythropoietin receptor-based dimerization assay was used to show that, upon complex assembly, the CD3epsilon chains of two CD3 heterodimers are arranged side-by-side in both the alphabeta and gammadeltaTCR-CD3 complexes. This system was also used to show that alphabetaTCRs can dimerize in the cell membrane and that mutating the unusual outer strands of the Calpha domain impairs this dimerization. Finally, we present data showing that, for CD4 T cells, the mutations that impair alphabetaTCR dimerization also alter ligand-induced calcium mobilization, TCR accumulation at the site of pMHC contact, and polarization toward the site of antigen contact. These data reveal a "functional-sidedness" to the alphabetaTCR constant region, with dimerization occurring on the side of the TCR opposite from where the CD3 heterodimers are located.
- Kuhns, M. S., & Davis, M. M. (2008). The safety on the TCR trigger. Cell, 135(4), 594-6.More infoIn this issue, Xu et al. (2008) provide evidence for a new mechanism of T cell receptor regulation. Prior to activation, basic residues in the cytoplasmic domain of the signaling subunits of the T cell receptor associate with the plasma membrane such that the key signaling tyrosines are sequestered in the bilayer.
- Huse, M., Klein, L. O., Girvin, A. T., Faraj, J. M., Li, Q., Kuhns, M. S., & Davis, M. M. (2007). Spatial and temporal dynamics of T cell receptor signaling with a photoactivatable agonist. Immunity, 27(1), 76-88.More infoThe precise timing of signals downstream of the T cell receptor (TCR) is poorly understood. To address this problem, we prepared major histocompatibility complexes containing an antigenic peptide that is biologically inert until exposed to ultraviolet (UV) light. UV irradiation of these complexes in contact with cognate T cells enabled the high-resolution temporal analysis of signaling. Phosphorylation of the LAT adaptor molecule was observed in 4 s, and diacylglycerol production and calcium flux was observed in 6-7 s. TCR activation also induced cytoskeletal polarization within 2 min. Antibody blockade of CD4 reduced the intensity of LAT phosphorylation and the speed of calcium flux. Furthermore, strong desensitization of diacylglycerol production, but not LAT phosphorylation, occurred shortly after TCR activation, suggesting that different molecular events play distinct signal-processing roles. These results establish the speed and localization of early signaling steps, and have important implications regarding the overall structure of the network.
- Kuhns, M. S., & Davis, M. M. (2007). Disruption of extracellular interactions impairs T cell receptor-CD3 complex stability and signaling. Immunity, 26(3), 357-69.More infoThe alphabeta T cell antigen receptor (TCR), in complex with the CD3deltavarepsilon, gammavarepsilon, and zetazeta signaling subunits, is the chief determinant for specific CD4(+) and CD8(+) T cell responses to self and foreign antigens. Although transmembrane domain charge interactions are critical for the assembly of the complex, the location of extracellular contacts between the TCR and CD3 subunits and their contributions to stability and signal transduction have not been defined. Here we used mutagenesis to demonstrate that the CD3deltavarepsilon and CD3gammavarepsilon subunits interact with the TCR via adjacent Calpha DE and Cbeta CC' loops, respectively. The TCR-CD3deltavarepsilon interactions helped stabilize CD3gammavarepsilon within the complex and were important for normal T cell and thymocyte responses to TCR engagement. These data demonstrate that extracellular TCR-CD3 subunit interactions contribute to the structural integrity and function of this multisubunit receptor.
- Huse, M., Lillemeier, B. F., Kuhns, M. S., Chen, D. S., & Davis, M. M. (2006). T cells use two directionally distinct pathways for cytokine secretion. Nature immunology, 7(3), 247-55.More infoActivated T helper cells produce many cytokines, some of which are secreted through the immunological synapse toward the antigen-presenting cell. Here we have used immunocytochemistry, live-cell imaging and a surface-mediated secretion assay to show that there are two cytokine export pathways in T helper cells. Some cytokines, including interleukin 2 and interferon-gamma, were secreted into the synapse, whereas others, including tumor necrosis factor and the chemokine CCL3 (MIP-1alpha), were released multidirectionally. Each secretion pathway was associated with different trafficking proteins, indicating that they are molecularly distinct processes. These data suggest that T helper cells release some cytokines into the immunological synapse to impart specific communication and others multidirectionally to promote inflammation and to establish chemokine gradients.
- Kuhns, M. S., Davis, M. M., & Garcia, K. C. (2006). Deconstructing the form and function of the TCR/CD3 complex. Immunity, 24(2), 133-9.More infoWhen T cells encounter antigens via the T cell antigen receptor (TCR), information about the quantity and quality of antigen engagement is relayed to the intracellular signal transduction machinery. This process is poorly understood. The TCR itself lacks a significant intracellular domain. Instead, it is associated with CD3 molecules that contain intracellular signaling domains that couple the TCR/CD3 complex to the downstream signaling machinery. The earliest events in TCR signaling must involve the transfer of information from the antigen binding TCR subunit to the CD3 signaling subunits of the TCR/CD3 complex. Elucidating the structural organization of the TCR with the associated CD3 signaling molecules is necessary for understanding the mechanism by which TCR engagement is coupled to activation. Here, we review the current state of our understanding of the structure and organization of the TCR/CD3 complex.
- Egen, J. G., Kuhns, M. S., & Allison, J. P. (2002). CTLA-4: new insights into its biological function and use in tumor immunotherapy. Nature immunology, 3(7), 611-8.More infoThe discovery of multiple costimulatory cell surface molecules that influence the course of T cell activation has increased our appreciation of the complexity of the T cell response. It remains clear, however, that CD28 and cytotoxic T lymphocyte antigen 4 (CTLA-4) are the critical costimulatory receptors that determine the early outcome of stimulation through the T cell antigen receptor (TCR). Details of how the T cell integrates TCR stimulation with the costimulatory signals of CD28 and the inhibitory signals of CTLA-4 remain to be established, but unique features of the cell biology of CTLA-4 provide important insights into its function. We summarize here recent findings that suggest a previously unrecognized role for CTLA-4 in the regulation of T cell responses. We also describe preclinical and clinical results that indicate manipulation of CTLA-4 has considerable promise as a strategy for the immunotherapy of cancer.
- Chambers, C. A., Kuhns, M. S., Egen, J. G., & Allison, J. P. (2001). CTLA-4-mediated inhibition in regulation of T cell responses: mechanisms and manipulation in tumor immunotherapy. Annual review of immunology, 19, 565-94.More infoThe T cell compartment of adaptive immunity provides vertebrates with the potential to survey for and respond specifically to an incredible diversity of antigens. The T cell repertoire must be carefully regulated to prevent unwanted responses to self. In the periphery, one important level of regulation is the action of costimulatory signals in concert with T cell antigen-receptor (TCR) signals to promote full T cell activation. The past few years have revealed that costimulation is quite complex, involving an integration of activating signals and inhibitory signals from CD28 and CTLA-4 molecules, respectively, with TCR signals to determine the outcome of a T cell's encounter with antigen. Newly emerging data suggest that inhibitory signals mediated by CTLA-4 not only can determine whether T cells become activated, but also can play a role in regulating the clonal representation in a polyclonal response. This review primarily focuses on the cellular and molecular mechanisms of regulation by CTLA-4 and its manipulation as a strategy for tumor immunotherapy.
- Kuhns, M. S., Epshteyn, V., Sobel, R. A., & Allison, J. P. (2000). Cytotoxic T lymphocyte antigen-4 (CTLA-4) regulates the size, reactivity, and function of a primed pool of CD4+ T cells. Proceedings of the National Academy of Sciences of the United States of America, 97(23), 12711-6.More infoWe examined how cytotoxic T lymphocyte antigen-4 (CTLA-4) regulates heterogeneous CD4(+) T cell responses by using experimental autoimmune encephalomyelitis (EAE), a CD4(+) T cell-mediated disease that is subject to regulation by CTLA-4. Disease incidence and severity were used as measures of in vivo CD4(+) T cell responses. The frequency, cytokine production, and reactivity of primed T cells were determined from animals immunized with proteolipid protein (PLP)-139-151 (disease agonist), PLP-Q (disease antagonist), or both peptides, and treated with control or anti-CTLA-4 antibody to analyze the responding population. CTLA-4 blockade exacerbated disease in PLP-139-151-primed animals and overcame disease antagonism in coimmunized animals, but did not permit disease induction in PLP-Q-primed animals. Experimental autoimmune encephalomyelitis enhancement was associated with increased frequencies of cytokine-producing cells and increased ratios of IFN-gamma to IL-4 secretors responsive to PLP-139-151. Priming with PLP-Q elicited IL-4 and IL-2, but not IFN-gamma secretors cross-reactive with PLP-139-151. Strikingly, CTLA-4 blockade was found to decrease rather than increase the frequencies of cross-reactive IL-4 and IL-2 secretors. Thus, CTLA-4 engagement limits the size, but increases the breadth, of reactivity of a primed pool of CD4(+) T cells, consequently regulating its function.
- Chambers, C. A., Kuhns, M. S., & Allison, J. P. (1999). Cytotoxic T lymphocyte antigen-4 (CTLA-4) regulates primary and secondary peptide-specific CD4(+) T cell responses. Proceedings of the National Academy of Sciences of the United States of America, 96(15), 8603-8.More infoCTLA-4-deficient mice develop a fatal lymphoproliferative disorder, characterized by polyclonal expansion of peripheral lymphocytes. To examine the effect of restricting the CD4(+) TCR repertoire on the phenotype of CTLA-4-deficient mice and to assess the influence of CTLA-4 on peptide-specific CD4(+) T cell responses in vitro, an MHC class II-restricted T cell receptor (AND TCR) transgene was introduced into the CTLA-4(-/-) animals. The expression of the AND TCR transgene by CD4(+) T cells delays but does not prevent the lymphoproliferation in the CTLA-4(-/-) mice. The CD4(+) T cells become preferentially activated and expand. Interestingly, young AND TCR(+) CTLA-4(-/-) mice carrying a null mutation in the rag-1 gene remain healthy and the T cells maintain a naive phenotype until later in life. We demonstrate that CTLA-4 regulates the peptide-specific proliferative response generated by naive and previously activated AND TCR(+) RAG(-/-) T cells in vitro. The absence of CTLA-4 also augments the responder frequency of cytokine-secreting AND TCR(+) RAG(-/-) T cells. These results demonstrate that CTLA-4 is a key regulator of peptide-specific CD4(+) T cell responses and support the model that CTLA-4 plays a differential role in maintaining T cell homeostasis of CD4(+) vs. CD8(+) T cells.