The kinetics of CD4+Foxp3+ T cell accumulation during a human cutaneous antigen-specific memory response in vivo

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Abstract
Naturally occurring CD4+CD25hiFoxp3+ Tregs (nTregs) are highly proliferative in blood. However, the kinetics of their accumulation and proliferation during a localized antigen-specific T cell response is currently unknown. To explore this, we used a human experimental system whereby tuberculin purified protein derivative (PPD) was injected into the skin and the local T cell response analyzed over time. The numbers of both CD4+Foxp3– (memory) and CD4+Foxp3+ (putative nTreg) T cells increased in parallel, with the 2 populations proliferating at the same relative rate. In contrast to CD4+Foxp3– T cell populations, skin CD4+Foxp3+ T cells expressed typical Treg markers (i.e., they were CD25hi, CD127lo, CD27+, and CD39+) and did not synthesize IL-2 or IFN-γ after restimulation in vitro, indicating that they were not recently activated effector cells. To determine whether CD4+Foxp3+ T cells in skin could be induced from memory CD4+ T cells, we expanded skin-derived memory CD4+ T cells in vitro and anergized them. These cells expressed high levels of CD25 and Foxp3 and suppressed the proliferation of skin-derived responder T cells to PPD challenge. Our data therefore demonstrate that memory and CD4+ Treg populations are regulated in tandem during a secondary antigenic response. Furthermore, it is possible to isolate effector CD4+ T cell populations from inflamed tissues and manipulate them to generate Tregs with the potential to suppress inflammatory responses.