Differential Requirements for CD4 in TCR-Ligand Interactions

The coreceptor molecule, CD4, plays an integral part in T cell activation; it is involved in both extracellular Ag recognition and intracellular signaling. We wanted to examine the functional role of CD4 in the recognition of agonist and altered peptide ligands (APLs). We generated two CD4-deficient T cell lines expressing well-characterized TCRs specific for Hb(64–76)/I-Ek. Although the responsiveness of the T cell lines to the agonist peptide was differently affected by the loss of CD4 expression, the recognition of APLs was in both cases dramatically reduced. Nearly full responsiveness to the agonist peptide was achieved by expression of a CD4 variant that did not associate with p56lck; however, the stimulation by APLs was only partially restored. Importantly, the expression of a CD4 variant in which domains interacting with MHC class II molecules have been mutated failed to restore the reactivity to all ligands. CD4-deficient T cells were able to be antagonized by APLs, indicating that CD4 was not required for antagonism. Overall, these findings support the concepts that CD4 is an integral part of the initial formation of the immunological synapse, and that the requirement for different CD4 functions in T cell activation varies depending upon the potency of the ligand.

Phospho-LAT-Independent Activation of the Ras-Mitogen-Activated Protein Kinase Pathway: A Differential Recruitment Model of TCR Partial Agonist Signaling

Stimulation of mature T cells with agonist ligands of the Ag receptor (TCR) causes rapid phosphorylation of tyrosine-based activation motifs in the intracellular portion of TCR-ζ and CD3 and activation of several intracellular signaling cascades. Coordinate activation of these pathways is dependent on Lck- and ZAP-70-mediated tyrosine phosphorylation of a 36-kDa linker for activation of T cells and subsequent recruitment of phospholipase C-γ1, Grb2-SOS, and SLP-76-vav. Here, we show that TCR partial agonist ligands can selectively activate one of these pathways, the Ras-mitogen-activated protein kinase pathway, by inducing recruitment of Grb2-SOS complexes to incompletely phosphorylated p21 phospho-TCR-ζ. This bypasses the need for activation of Lck and ZAP-70, and for phosphorylation of the linker for activation of T cells to activate Ras. We propose a general model in which differential recruitment of activating complexes away from transmembrane linker proteins may determine selective activation of a given signaling pathway.

Differential CD3ζ Phosphorylation Is Not Required for the Induction of T Cell Antagonism by Altered Peptide Ligands

T cells recognize foreign Ags in the form of short peptides bound to MHC molecules. Ligation of the TCR:CD3 complex gives rise to the generation of two tyrosine-phosphorylated forms of the CD3 ζ-chain, pp21 and pp23. Replacement of residues in MHC-bound peptides that alter its recognition by the TCR can generate altered peptide ligands (APL) that antagonize T cell responses to the original agonist peptide, leading to altered T cell function and anergy. This biological process has been linked to differential CD3ζ phosphorylation and generation of only the pp21 phospho-species. Here, we show that T cells expressing CD3ζ mutants, which cannot be phosphorylated, exhibit a 5-fold reduction in IL-2 production and a 30-fold reduction in sensitivity following stimulation with an agonist peptide. However, these T cells are still strongly antagonized by APL. These data demonstrate that: 1) the threshold required for an APL to block a response is much lower than for an agonist peptide to induce a response, 2) CD3ζ is required for full agonist but not antagonist responses, and 3) differential CD3ζ phosphorylation is not a prerequisite for T cell antagonism.

Anergic CD8+ T Cells Can Persist and Function In Vivo

Using a mouse model system, we demonstrate that anergic CD8+ T cells can persist and retain some functional capabilities in vivo, even after the induction of tolerance. In TCR Vβ5 transgenic mice, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction. CD8low cells are characterized by an activated phenotype, are functionally compromised in vitro, and are slated for deletion in vivo. We now demonstrate that CD8low cells derive from a proliferative compartment, but do not divide in vivo. CD8low cells persist in vivo with a t1/2 of 3–5 days, in contrast to their in vitro t1/2 of 0.5–1 day. During this unexpectedly long in vivo life span, CD8low cells are capable of producing IFN-γ in vivo despite their inability to proliferate or to kill target cells in vitro. CD8low cells also accumulate at sites of inflammation, where they produce IFN-γ. Therefore, rather than withdrawing from the pool of functional CD8+ T cells, anergic CD8low cells retain a potential regulatory role despite losing their capacity to proliferate. The ability of anergic cells to persist and function in vivo adds another level of complexity to the process of tolerance induction in the lymphoid periphery.

Serial TCR Engagement and Down-Modulation by Peptide:MHC Molecule Ligands: Relationship to the Quality of Individual TCR Signaling Events

In the present study, we examined the relationships among quantitative aspects of TCR engagement as measured by receptor down-modulation, functional responses, and biochemical signaling events using both mouse and human T cell clones. For T cells from both species, ligands that are more potent in inducing functional responses promote TCR down-modulation more efficiently than weaker ligands. At low ligand density, the number of down-modulated TCR exceeds the number of available ligands by as much as 80–100:1 in the optimal human case, confirming the previous description of serial ligand engagement of TCR (Valitutti, et al. 1995. Nature 375:148–151). A previously unappreciated relationship involving TCR down-modulation, the pattern of proximal TCR signaling, and the extent of serial engagement was revealed by analyzing different ligands for the same TCR. Functionally, more potent ligands induce a higher proportion of fully tyrosine phosphorylated ζ-chains and a greater amount of phosphorylated ZAP-70 than less potent ligands, and the number of TCR down-modulated per available ligand is higher with ligands showing this full agonist-like pattern. The large number of receptors showing partial ζ phosphorylation following exposure to weak ligands indicates that the true extent of TCR engagement and signaling, and thus the amount of sequential engagement, is underestimated by measurement of TCR down-modulation alone, which depends on full receptor activation. These data provide new insight into T cell activation by revealing a clear relationship among intrinsic ligand quality, signal amplification by serial engagement, functional T cell responses, and observable TCR clearance from the cell surface.

Termination of Peripheral Tolerance to a T Cell Epitope by Heteroclitic Antigen Analogues

Treating mice with an immunodominant T cell epitope from moth cytochrome c (MCC88–103) can induce T cell unresponsiveness under certain conditions of administration. In this report, we determined whether T cell tolerance to MCC88–103 in adult animals can be overcome by immunization with cross-reactive analogues of the tolerizing Ag. A panel of analogues of the tolerogen were tested for their capacity to terminate the tolerant state following in vivo immunization. As analyzed by their stimulatory capacity for a representative MCC88–103-specific T cell clone, this panel covered a wide range of cross-reactivity, including nonantigenic, antagonistic, weakly, and strongly antigenic peptides. Interestingly, only heteroclitic analogues, as measured in vitro by their enhanced antigenicity for the T cell clone that was specific for MCC88–103, were capable of breaking tolerance. Thus, an immune response to the cross-reactive, heteroclitic analogues of tolerized self Ags may represent a mechanism by which Ag molecular mimicry operates.

Two Distinct Pathways Exist for Down-Regulation of the TCR

TCR down-regulation plays an important role in modulating T cell responses both during T cell development and in mature T cells. Down-regulation of the TCR is induced by engagement of the TCR by specific ligands and/or by activation of protein kinase C (PKC). We report here that ligand- and PKC-induced TCR down-regulation is mediated by two distinct, independent mechanisms. Ligand-induced TCR down-regulation is dependent on the protein tyrosine kinases p56lck and p59fyn but independent of PKC and the CD3γ leucine-based (L-based) internalization motif. In contrast, PKC-induced TCR down-regulation is dependent on the CD3γ L-based internalization motif but independent of p56lck and p59fyn. Finally, our data indicate that in the absence of TCR ligation, TCR expression levels can be finely regulated via the CD3γ L-based motif by the balance between PKC and serine/threonine protein phosphatase activities. Such a TCR ligation-independent regulation of TCR expression levels could probably be important in determining the activation threshold of T cells in their encounter with APC.

In Vivo Expression of a TCR Antagonist: T Cells Escape Central Tolerance But Are Antagonized in the Periphery

Transgenic 3.L2 T cells are stimulated by Hb(64–76)/I-Ek and are positively selected on I-Ek plus self-peptides. To this pool of self-peptides we have added a single, well-defined 3.L2 TCR antagonist (A72) in vivo. We find that mice expressing both the 3.L2 TCR and A72 have a minimal loss of T cells expressing the clonotypic TCR in the thymus and spleen. Importantly, the proliferative response of 3.L2 × A72 splenocytes is significantly reduced compared with splenocytes from 3.L2 mice. This reduced response can be attributed to peripheral antagonism. Thus we have identified a new class of self-ligands whose predominant effect is constitutive peripheral antagonism rather than negative selection. The net effect of these ligands is to avoid potential self-reactivity while maintaining as large a repertoire as possible.

Modulation of Naive CD4 T Cell Activation with Altered Peptide Ligands: The Nature of the Peptide and Presentation in the Context of Costimulation Are Critical for a Sustained Response

Altered peptide ligands containing single amino acid substitutions have the potential to be used for modulating immune function. Using a panel of moth cytochrome c peptides, we demonstrate that different phases of naive CD4 T cell response are alternately modulated depending on altered peptide ligand dose and accessory molecule expression by APC. Weak agonists presented at high concentration, and with costimulation, efficiently induced early phase naive T cell activation as assessed by IL-2R/CD69 expression, but could only promote sufficient IL-2 for a short-lived proliferative response. In contrast, strong agonists and heteroclitic peptides induced early phase T cell activation even at low concentrations with costimulation, and allowed sustained IL-2 secretion and proliferation. In the absence of accessory molecule help, early and late phase activation was impaired with weak agonists, whereas strong agonists partially compensated for a lack of costimulation for early phase activation, and also promoted enhanced IL-2 with sustained proliferation. These studies support the hypothesis that the naive T cell response will be determined by the balance between provision of accessory molecule help and the affinity of peptide/MHC complexes for individual TCRs, and suggest that extended IL-2 production is the main facet of naive CD4 activation that is affected by altering the nature of the peptide.

Inhibition of an In Vitro CD4+ T Cell Alloresponse Using Altered Peptide Ligands

In this study, we explore the potential of altered peptide ligands (APLs) to modulate the alloresponse of CD4+ T cells using elements of the murine hemoglobin (Hb) Ag model. We first demonstrated that the T cell 2.102, specific for the Hb(64-76)/I-Ek complex, was alloreactive against splenocytes of the H-2p haplotype. Using Ab-blocking and transfection experiments, we further showed that this alloreactivity was restricted to the class II molecule I-Ep. We tested a panel of APLs previously shown to antagonize the Hb response of 2.102 and found that these peptides could also effectively inhibit the alloresponse to I-Ep. Importantly, these peptides were able to antagonize the alloresponse of naive T cells derived from mice transgenic for the 2.102 TCR, as well as Th1 and Th2 cell lines. The antagonism required the presence of both I-Ep and I-Ek on the same APC. Our study demonstrates the effectiveness of APLs to antagonize the primary alloresponse of specific T cells and provides a basis for the development of immunotherapeutics for use in transplantation and immune-mediated diseases.