TCR-signaling strength generally correlates with peptide-MHC binding affinity; however, exceptions exist. We find high-affinity, yet non-stimulatory, interactions occur with high frequency in the human T cell repertoire. Here, we studied human TCRs that are refractory to activation by pMHC ligands despite robust binding. Analysis of 3D affinity, 2D dwell time, and crystal structures of stimulatory versus non-stimulatory TCR-pMHC interactions failed to account for their different signaling outcomes. Using yeast pMHC display, we identified peptide agonists of a formerly non-responsive TCR. Single-molecule force measurements demonstrated the emergence of catch bonds in the activating TCR-pMHC interactions, correlating with exclusion of CD45 from the TCR-APC contact site. Molecular dynamics simulations of TCR-pMHC disengagement distinguished agonist from non-agonist ligands based on the acquisition of catch bonds within the TCR-pMHC interface. The isolation of catch bonds as a parameter mediating the coupling of TCR binding and signaling has important implications for TCR and antigen engineering for immunotherapy.

In response to infection, naïve CD4 T cells differentiate into two subpopulations: T follicular helper (T) cells, which support B cell antibody production, and non-T cells, which enhance innate immune cell functions. Interleukin-2 (IL-2), the major cytokine produced by naïve T cells, plays an important role in the developmental divergence of these populations. However, the relationship between IL-2 production and fate determination remains unclear. Using reporter mice, we found that differential production of IL-2 by naïve CD4 T cells defined precursors fated for different immune functions. IL-2 producers, which were fated to become T cells, delivered IL-2 to nonproducers destined to become non-T cells. Because IL-2 production was limited to cells receiving the strongest T cell receptor (TCR) signals, a direct link between TCR-signal strength, IL-2 production, and T cell fate determination has been established.

Background Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid produced by mast cells (MC) upon cross-linking of their high affinity receptors for IgE by antigen (Ag) that can amplify MC responses by binding to its S1P receptors. Acute MC-dependent allergic reaction can lead to systemic shock but the early events of its development in lung tissues have not been investigated, and S1P functions in the onset of allergic processes remain to be examined. Objective We used a highly specific neutralizing anti-S1P antibody (mAb) and an S1P receptor 2 (S1PR2) antagonist, JTE-013, to study S1P and S1PR2 signaling contributions to MC- and IgE-dependent airway allergic responses in mice within minutes after Ag challenge. Methods Allergic reaction was triggered by a single intraperitoneal (i.p.) dose of Ag in sensitized mice pre-treated i.p. with anti-S1P or isotype control mAb, or JTE-013 or vehicle prior to Ag challenge. Results Kinetics experiments revealed early pulmonary infiltration of mostly T cells around blood vessels of sensitized mice 20 minutes post-Ag exposure. Pre-treatment with anti-S1P mAb inhibited in vitro MC activation, as well as in vivo development of airway infiltration and MC activation, reducing serum levels of histamine, cytokines and the chemokines MCP-1/CCL2, MIP-1α/CCL3 and RANTES/CCL5. S1PR2 antagonism or deficiency, or MC deficiency recapitulated these results. Both in vitro and in vivo experiments demonstrated MC S1PR2 dependency for chemokine release and the necessity for signal transducer and activator of transcription 3 (Stat3) activation. Conclusion Activation of S1PR2 by S1P and downstream Stat3 signaling in MC regulate early T cell recruitment to antigen-challenged lungs by chemokine production.

T cell receptor (TCR) recognition of peptide–major histocompatibility complexes (pMHCs) is characterized by a highly conserved docking polarity. Whether this polarity is driven by recognition or signaling constraints remains unclear. Using “reversed-docking” TCRβ-variable (TRBV) 17+ TCRs from the naïve mouse CD8+ T cell repertoire that recognizes the H-2Db–NP366 epitope, we demonstrate that their inability to support T cell activation and in vivo recruitment is a direct consequence of reversed docking polarity and not TCR–pMHCI binding or clustering characteristics. Canonical TCR–pMHCI docking optimally localizes CD8/Lck to the CD3 complex, which is prevented by reversed TCR–pMHCI polarity. The requirement for canonical docking was circumvented by dissociating Lck from CD8. Thus, the consensus TCR–pMHC docking topology is mandated by T cell signaling constraints.

Adoptive cell therapy using engineered T cell receptors (TCRs) is a promising approach for targeting cancer antigens, but tumor-reactive TCRs are often weakly responsive to their target ligands, peptide–major histocompatibility complexes (pMHCs). Affinity-matured TCRs can enhance the efficacy of TCR–T cell therapy but can also cross-react with off-target antigens, resulting in organ immunopathology. We developed an alternative strategy to isolate TCR mutants that exhibited high activation signals coupled with low-affinity pMHC binding through the acquisition of catch bonds. Engineered analogs of a tumor antigen MAGE-A3–specific TCR maintained physiological affinities while exhibiting enhanced target killing potency and undetectable cross-reactivity, compared with a high-affinity clinically tested TCR that exhibited lethal cross-reactivity with a cardiac antigen. Catch bond engineering is a biophysically based strategy to tune high-sensitivity TCRs for T cell therapy with reduced potential for adverse cross-reactivity.

Lactic acid (LA) is present in tumors, asthma, and wound healing, environments with elevated IL-33 and mast cell infiltration. While IL-33 is a potent mast cell activator, how LA affects IL-33-mediated mast cell function is unknown. To investigate this, mouse bone marrow-derived mast cells (BMMC) were cultured with or without LA and activated with IL-33. LA reduced IL-33-mediated cytokine and chemokine production. Using inhibitors for monocarboxylate transporters (MCT) or replacing LA with sodium lactate revealed that LA effects are MCT-1- and pH-dependent. LA selectively altered IL-33 signaling, suppressing TAK1, JNK, ERK, and NFκB phosphorylation, but not p38 phosphorylation. LA effects in other contexts have been linked to HIF-1α, which was enhanced in BMMC treated with LA. Since HIF-1α has been shown to regulate the microRNA miR-155 in other systems, LA effects on miR-155-5p and -3p species were measured. In fact, LA selectively suppressed miR-155-5p in a HIF-1α-dependent manner. Moreover, overexpressing miR-155-5p, but not miR-155-3p, abolished LA effects on IL-33-induced cytokine production. These in vitro effects of reducing cytokines were consistent in vivo, since LA injected intraperitoneally into C57BL/6 mice suppressed IL-33-induced plasma cytokine levels. Lastly, IL-33 effects on primary human mast cells were suppressed by LA in an MCT-dependent manner. Our data demonstrate that LA, present in inflammatory and malignant microenvironments, can alter mast cell behavior to suppress inflammation.

We previously demonstrated that Transforming Growth Factor (TGF) β1 suppresses IgE-mediated signaling in human and mouse mast cells in vitro, an effect that correlated with decreased expression of the high affinity IgE receptor, FcεRI. The in vivo effects of TGFβ1 and the means by which it suppresses mast cells have been less clear. The current study shows that TGFβ1 suppresses FcεRI and c-Kit expression in vivo. By examining changes in cytokine production concurrent with FcεRI expression, we found that TGFβ1 suppresses TNF production independent of FcεRI levels. Rather, IgE-mediated signaling was altered. TGFβ1 significantly reduced expression of Fyn and Stat5, proteins critical for cytokine induction. These changes may partly explain the effects of TGFβ1, since Stat5B overexpression blocked TGF-mediated suppression of IgE-induced cytokine production. We also found that Stat5B is required for mast cell migration toward SCF, and that TGFβ1 reduced this migration. We found evidence that genetic background may alter TGF responses. TGFβ1 greatly reduced mast cell numbers in Th1-prone C57BL/6 but not Th2-prone 129/Sv mice. Furthermore, TGFβ1 did not suppress IgE-induced cytokine release, and increased c-Kit-mediated migration in 129/Sv mast cells. These data correlated with high basal Fyn and Stat5 expression in 129/Sv cells, which was not reduced by TGFβ1 treatment. Finally, primary human mast cell populations also showed variable sensitivity to TGFβ1-mediated changes in Stat5 and IgE-mediated IL-6 secretion. We propose that TGFβ1 regulates mast cell homeostasis, and that this feedback suppression may be dependent upon genetic context, predisposing some individuals to atopic disease.

The LCMV GP33 CD8 epitope has long been one of the most widely used antigens in viral immunology. Of note, almost all of the in vitro analyses of CD8 T cell responses to this epitope make use of an altered peptide ligand (APL) in which the cysteine from the original 9-mer peptide (KAVYNFATC) is substituted by a methionine at position 41 (KAVYNFATM). In addition, it is possible that the antigen processed during natural LCMV infection is an 11-mer peptide (KAVYNFATCGI) rather than the widely used 9-mer. Although previous affinity measurements using purified proteins for these antigen variants revealed minimal differences, we applied highly sensitive two dimensional (2D) biophysical based techniques to further dissect TCR interaction with these closely related GP33 variants. The kinetic analyses of affinity provided by the 2D micropipette adhesion frequency assay (2D-MP) and bond lifetime under force analyzed using a biomembrane force probe (BFP) revealed significant differences between 41M, 41C and the 11-mer 41CGI antigen. We found a hierarchy in 2D affinity as 41M peptide displayed augmented TCR 2D affinity compared to 41C and 41CGI. These differences were also maintained in the presence of CD8 coreceptor and when analysis of total TCR:pMHC and CD8:pMHC bonds were considered. Moreover, the three ligands displayed dramatic differences in the bond lifetimes generated under force, in particular the 41CGI variant with the lowest 2D affinity demonstrated a 15-fold synergistic contribution of the CD8 coreceptor to overall bond lifetime. Our analyses emphasize the sensitivity of single cell and single bond 2D kinetic measurements in distinguishing between related agonist peptides.