Heterogeneity in FoxP3- and GARP/LAP-Expressing T Regulatory Cells in an HLA Class II Transgenic Murine Model of Necrotizing Soft Tissue Infections by Group A Streptococcus.

Invasive group A streptococcus (GAS) infections include necrotizing soft tissue infections (NSTI) and streptococcal toxic shock syndrome (STSS). We have previously shown that host HLA class II allelic variations determine the risk for necrotizing fasciitis (NF), a dominant subgroup of NSTI, and STSS by modulating responses to GAS superantigens (SAgs). SAgs are pivotal mediators of uncontrolled T-cell activation, triggering a proinflammatory cytokine storm in the host. FoxP3-expressing CD4+ CD25+ T regulatory cells (Tregs) comprise phenotypically and functionally heterogeneous subsets with a profound ability to suppress inflammatory responses. Specifically, activated Tregs, which express glycoprotein A repetitions predominant (GARP) and display latent transforming growth factor β1 (TGF-β1) complexes (latency-associated peptide [LAP]), exhibit strong immunosuppressive functions. The significance of Tregs that may participate in suppressing inflammatory responses during NSTI is unknown. Here, we phenotypically characterized FoxP3/GARP/LAP-expressing Tregs in GAS-infected or SAg (SmeZ)-stimulated splenocytes from transgenic (tg) mice expressing human HLA-II DRB1*15 (DR15 allele associated with nonsevere NF/STSS-protective responses) or DRB1*0402/DQB1*0302 (DR4/DQ8 alleles associated with neutral risk for combined NF/STSS). We demonstrated both in vivo and in vitro that the neutral-risk allele upregulates expression of CD4+ CD25+ activated effector T cells, with a significantly lower frequency of Foxp3+/GARP+ LAP- but higher frequency of Foxp3- LAP+ Tregs than seen with the protective allele. Additional in vitro studies revealed that the presentation of SmeZ by the neutral-risk allele significantly increases proliferation and expression of effector cytokines gamma interferon (IFN-γ) and interleukin-2 (IL-2) and upregulates CD4+ CD25+ T cell receptors (TCRs) carrying specific Vβ 11 chain (TCRVβ11+) T cells and Th1 transcription factor Tbx21 mRNA levels. Our data suggest that neutral-risk alleles may drive Th1 differentiation while attenuating the induction of Tregs associated with suppressive function.