Peripheral “CD8 Tuning” Dynamically Modulates the Size and Responsiveness of an Antigen-Specific T Cell Pool In Vivo

Abstract

In this study, we suggest that CD8 levels on T cells are not static, but can change and, as a result, modulate CD8+ T cell responses. We describe three models of CD8 modulation using novel weak-agonist (K1A) and super-agonist (C2A) altered peptide ligands of the HY smcy peptide. First, we used peripheral nonresponsive CD8low T cells produced after peripheral HY-Db MHC class I tetramer stimulation of female HY TCR transgenic and wild-type mice. Second, we used genetically lowered CD8int T cells from heterozygote CD8+/0 mice. Finally, we used pre-existing nonresponsive CD8low T cells from male HY TCR transgenic mice. In CD8low and CD8high mice, presence of a lower level of CD8 greatly decreased the avidity of the peptide-MHC for HY TCR as reflected by avidity (KD) and dissociation constant (T1/2) measurements. All three models demonstrated that lowering CD8 levels resulted in the requirement for a higher avidity peptide-MHC interaction with the TCR to respond equivalently to unmanipulated CD8high T cells of the same specificity. Additionally, direct injections of wild-type HY-Db and C2A-Db tetramers into female HY TCR or female B6 mice induced a high frequency of peripheral nonresponsive CD8low T cells, yet C2A-Db was superior in inducing a primed CD8+CD44+ memory population. The ability to dynamically modulate the size and responsiveness of an Ag-specific T cell pool by “CD8 tuning” of the T cell during the early phases of an immune response has important implications for the balance of responsiveness, memory, and tolerance.

Low‐avidity CD8lo T cells induced by incomplete antigen stimulation in vivo regulate naive higher avidity CD8hi T cell responses to the same antigen

“…3a). Neither CD8 + T cells from untreated nor lower avidity K1A-D b -treated mice were able to suppress naive CD8 + T cell responses in vitro, confirming our previous results that a minimum avidity threshold is required for generation of CD8 lo T cells [17], and suggesting this is also the case for generation of suppressive ability.To confirm that the regulatory function lies within the CD8 lo T cell population, we further sorted purified CD8 + T cells into CD8 hi and CD8 lo T cells from tetramertreated mice. Following irradiation, only the CD8 lo T cells from HY-D b treated groups suppressed the response Figure 2.…”

“…We argue that these observations are CD8 + examples of the peripheral dynamic T cell "avidity regulation" model, in which T cell clones with certain avidity for cognate antigen are selectively suppressed [17,21,22], in this case by stable down-regulation of CD8 expression, which greatly impacts the ability of the CD8 + T cell to respond to low avidity antigens [17]. Here we describe that such CD8 lo T cells, despite being significantly nonresponsive to antigen per se [17][18][19][20] can also mediate dose-dependent suppression of naive, but not memory, CD8 + T cell proliferation to HY antigen in vitro. When adoptively transferred to naive female B6 recipients, HYspecific CD8 lo T cells prolong male skin graft survival in vivo.…”

“…Fig. 3a shows the response of naive HY TCR transgenic CD8 + T cells to HY antigen to which we added purified CD8 + T cells from untreated HY TCR mice, wild-type HY-D b -treated mice or control weak agonist K1A HY-D b tetramer-treated mice, which does not generate CD8 lo T cells in vivo [17]. The putative regulatory cells were gamma-irradiated to block their proliferation allowing us to measure only their regulatory potential in vitro.…”

Induced peripheral regulatory T cells: The family grows larger

“…Previous findings from the same group have demonstrated that HY-specific TCR transgenic cells with low CD8 expression levels respond with low avidity to cells presenting male antigens [24]. Consequently, the authors speculate that the CD8 low Treg that are induced after immunization with HY tetramers respond less well to normal levels of MHC class I/peptide on grafted male cells and that this feature provides, somehow, an explanation for their regulatory properties.…”

“…One possible explanation could be the different balance between activating and inhibitory influences in the two situations. In memory T cells, the threshold for stimulation is much lower than in naive T cells [24], which means that memory T cells will be more prone to execute their effector functions at lower levels of stimulation. In such a situation, the immunoregulatory molecules derived from Treg will perhaps not be present in quantitatively large enough numbers to counteract the triggering pathways.…”