Releasing CD8⁺ Treg Mediated Suppresssion Enhances Anti-Viral Immune Response

Recent findings have shown that a small subset of IL-15 dependent CD8⁺ regulatory T cells is essential for maintenance of self- tolerance and prevention of autoimmune disease in mice (Kim et al., Nature 2010). These CD8⁺ T cells target CD4⁺ follicular T helper (T_FH) cells through recognition of the murine class Ib MHC molecule Qa-1 (HLA-E in man), resulting in perforin-dependent elimination of target cells and diminished antibody production in the steady state and during disease. This analysis was based on generation of Qa-1 knock-in mice (D227K mice) that harbor a single Qa-1 D→K amino acid exchange point mutation at position 227 that abrogates binding of Qa-1/peptide to the CD8/TCR complex. B6.Qa-1 D227K mutant mice develop severe autoimmune disease marked by generation of autoantibodies to multiple tissues, lymphocyte infiltration into non-lymphoid tissues and lethal glomerulonephritis.

Qa-1-restricted CD8⁺ Treg are characterized by the CD44⁺CD122⁺Ly49⁺ phenotype (Kim et al., PNAS 2011). Here, we analyzed the contribution of CD8⁺ Treg to modulation of the anti-viral immune response. Virus-specific CD8⁺ cytotoxic T cells are of central importance for successful control of the Lymphocytic Choriomeningitis Virus (LCMV). LCMV clone 13, however, a genetic variant of LCMV Armstrong, persists in the host and chronic antigen exposure leads to exhaustion of CD8⁺ T cells and continuous tissue inflammation. The contribution of CD8⁺ Treg in the anti-viral immune response to acute and chronic viral infection remained elusive so far.

We found that CD8⁺ Treg not only control self-tolerance but also diminish the immune response to viral infection. By comparing wild-type and D227K mutant mice after infection with LCMV Armstrong or LCMV clone 13, we observed in both cases reduced effector CD8⁺ T cell responses. This was true for polyclonal CD44⁺CD62L^– CD8⁺ T cells as well as LCMV-specific gp33⁺ effector CD8⁺ T cells. During acute infection KLRG1⁺CD127^-CD44⁺CD62L^- cells (short-lived effector CD8⁺ cells) (Joshi et al., Immunity 2007) were particularly diminished as well as effector cytokines in wild-type mice compared to D227K mice. In contrast, increased effector responses in D227K mice resulted in enhanced control of virus and reduced inflammation of tissues.

During chronic infection with LCMV, wild-type mice become severely ill and present with a pronounced clinical phenotype. Increased effector CD8⁺ T cell immune responses in D227K mice resulted in dramatic alleviation of disease. During late stage of chronic infection, D227K mice showed enhanced virus control and reduced tissue pathology compared with wild-type mice. Interestingly, expression of inhibitory receptors such as PD-1, 2B4 and LAG3 were increased in wild-type mice whereas activating receptors such as NKG2D and KLRG1 were increased in D227K mice, resulting in a memory phenotype in D227K mice compared with exhausted CD8⁺ T cells in wild-type mice. Adoptive transfer experiments revealed that CD8⁺ Treg directly suppress CD8⁺ target cells and thereby inhibit induction of a robust anti-viral response.

Taken together, we show that Qa-1-restricted CD8⁺ Treg have a direct inhibitory effect on effector CD8⁺ T cells during acute and chronic viral infection, resulting in a more violent disease and diminished recovery. These data suggest that depletion or inactivation of CD8⁺ Treg represents a potentially effective strategy to enhance anti-viral immunity.