CD4+ T-cell responses to self-peptide--MHC

Competition for refueling rather than cyclic reentry initiation evident in germinal centers

Antibody affinity maturation occurs in germinal centers (GCs) through iterative rounds of somatic hypermutation and proliferation in dark zones (DZs) and selection in light zones (LZs). GC B cells exit cell cycle a number of hours before entering LZs; therefore, continued participation in responses requires that they subsequently reenter cell cycle and move back to DZs, a process known as cyclic reentry. Affinity enhancements are thought to arise by B cells having to compete to initiate cyclic reentry each time they enter LZs, with T cell help being a major determinant; however, direct proof is lacking. Using Fucci2 mice, we confirmed an association between B cell receptor affinity and the first step of cyclic reentry, S phase initiation from a resting LZ state. However, neither T cell ablation nor MHCII deletion prevented resting LZ cells from reentering cell cycle, and this late G₁-S transition was also not detectably restricted by competition. In contrast, using BATF induction as exemplar, we found that T cells "refueled" LZ cells in an affinity-dependent manner that was limited by both competition and cells' intrinsic antigen-acquiring abilities. Therefore, cyclic reentry initiation and B cell refueling are independently regulated in GCs, which may contribute to permitting cells of different competencies to be sustained alongside each other and allow T cell support to be provided across a dynamic range commensurate with affinity. We speculate that this less binary selection mechanism could help GCs nurture complex antibody maturation pathways and support the clonal diversity required for countering fast-evolving pathogens.

Naive CD4 t cell proliferation is controlled by mammalian target of rapamycin regulation of GRAIL expression

In this study, we demonstrate that the E3 ubiquitin ligase gene related to anergy in lymphocytes (GRAIL) is expressed in quiescent naive mouse and human CD4 T cells and has a functional role in inhibiting naive T cell proliferation. Following TCR engagement, CD28 costimulation results in the expression of IL-2 whose signaling through its receptor activates the Akt-mammalian target of rapamycin (mTOR) pathway. Activation of mTOR allows selective mRNA translation, including the epistatic regulator of GRAIL, Otubain-1 (Otub1), whose expression results in the degradation of GRAIL and allows T cell proliferation. The activation of mTOR appears to be the critical component of IL-2R signaling regulating GRAIL expression. CTLA4-Ig treatment blocks CD28 costimulation and resultant IL-2 expression, whereas rapamycin and anti-IL-2 treatment block mTOR activation downstream of IL-2R signaling. Thus, all three of these biotherapeutics inhibit mTOR-dependent translation of mRNA transcripts, resulting in blockade of Otub1 expression, maintenance of GRAIL, and inhibition of CD4 T cell proliferation. These observations provide a mechanistic pathway sequentially linking CD28 costimulation, IL-2R signaling, and mTOR activation as important requirements for naive CD4 T cell proliferation through the regulation of Otub1 and GRAIL expression. Our findings also extend the role of GRAIL beyond anergy induction and maintenance, suggesting that endogenous GRAIL regulates general cell cycle and proliferation of primary naive CD4 T cells.

MHC-dependent desensitization of intrinsic anti-self reactivity

The survival of naive T cells is compromised in the absence of molecules encoded by the major histocompatibility complex (MHC) while antigen-experienced T cells survive. We hypothesized that survival pressures in an in vivo, MHC-deficient environment would permit enrichment of less frequent antigen-experienced autoreactive cells at the expense of the majority of antigen naive T cells. To test this hypothesis, we generated MHC class I- and class II-deficient mice in NOD and C57Bl/6 (B6) backgrounds, and examined the capacity of adoptively transferred autoimmune-prone NOD T cells, or non-autoimmune prone naive B6 T cells, respectively, to reject transplanted wild-type pancreatic islets or transplantable tumors in the MHC-deficient mice. In the MHC-deficient environment, CD4 T cells acquired self-hostile properties (islet rejection and tumor invasion) that were independent from their genetic propensity for autoreactivity, while CD8 T cells required appropriate prior exposure to antigen in order to survive and function (reject tumor) in this environment; however, disengagement of Tob1, a negative regulator of proliferation, led to a reverse phenotype with regard to persistence of CD4 and CD8 T cells in the MHC-deficient environment. Our data suggest that self-peptide/MHC interactions have dual roles to facilitate survival and restrain autoreactivity, thus acting as integral components of an intrinsic network of negative regulation that maintains tolerance.

Cutting edge: differential self-peptide/MHC requirement for maintaining CD8 T cell function versus homeostatic proliferation

Memory T cells do not require self-peptide/MHC (spMHC) complexes to survive long term in vivo. However, memory CD4 T cells lose the ability to reject skin grafts when transiently placed in an environment in which these low-level TCR stimulations are absent. Whether or not spMHC alters the ability of CD8 T cells to respond to stimulation in vivo remains unknown. Here, we show that memory CD8 T cells retain the ability to respond to dendritic cell-mediated stimulation after adoptive transfer into either TAP(-/-) (MHC class I-deficient) or wild-type mice. Surprisingly, naive CD8 T cells, which fail to undergo homeostatic proliferation and erode in number in the absence of MHC class I, also retain the ability to respond to dendritic cell-mediated antigenic stimulation for at least 1 wk after transfer into TAP(-/-) mice. These findings suggest a differential requirement for spMHC signals for maintenance of CD8 T cell function and homeostatic proliferation.

Loss of CD8 and TCR binding to Class I MHC ligands following T cell activation

The capacity of T cells to bind peptide/MHC ligands changes with T cell development and differentiation. Here we study changes in peptide/MHC multimer binding following T cell activation. Surprisingly, T cell activation caused a marked reduction in specific peptide/MHC Class I multimer binding, which was distinct from transient TCR down-regulation, and was especially dramatic for engagement with low-affinity peptide/MHC ligands. Direct CD8-Class I interactions were also profoundly and rapidly impaired following T cell stimulation, even though surface CD8alpha and CD8beta levels were unchanged after activation, suggesting that decreased CD8 co-receptor binding contributes to this effect. Finally, we show that enzymatic desialylation restores much of the multimer binding on activated T cells, suggesting that altered glycosylation may inhibit TCR/CD8 binding to peptide/MHC ligands. These radical changes in activated T cells' ability to perceive peptide/MHC ligands may contribute to selective outgrowth of clones with high affinity for the stimulatory ligand.