Functional Separation of GVHD-Derived and Naturally Occurring CD4+CD25+ Regulatory T Cells.

Graft versus host disease (GVHD) is mediated by mature alloreactive donor T cells. Upon activation, alloreactive CD4+ T cells upregulate CD134 (OX40), a member of the TNF receptor superfamily. Using a rat hapolidentical parent into F1 model of GVHD, we recently reported that CD25 expression stratifies these CD4+134+ T cells into two alloreactive T cell subsets (Biol Blood Marrow Transplant. 2004 May;10(5):298-309; results summarized in the table below).

In the current study we investigated the immune regulatory potential of the GVHD-associated CD4+CD25+ T cell subset. Cocultivation of the two GVHD-associated CD4+134+ T cell subsets revealed that the GVHD-derived CD4+CD25+ T cells suppressed, in a dose dependent manner, alloantigen-induced proliferation of the CD4+CD25− T cell subset. As this observation is similar to the suppressive activity associated with naturally occurring CD4+CD25+ regulatory T cells, cells implicated in the inhibition or suppression of a wide range of immune responses, it suggested that GVHD-associated CD4+CD25+ T cells play a regulatory role in GVHD. However, in further contrasting the biological properties of GVHD-associated CD4+CD25+ T cells with those of naturally occurring CD4+CD25+ regulatory T cells, substantive differences in these two types of regulatory T cells became apparent. At the phenotypic level, GVHD-associated regulatory T cells expressed higher levels of MHC Class II and lower levels of CD45RC and CD62L, suggesting different stages of activation. GVHD-associated and naturally occurring regulatory T cells also differed functionally. While naturally occurring regulatory T cells suppressed primary mixed lymphocyte cultures, GVHD-derived CD4+CD25+ regulatory T cells provided a potent proliferative advantage for responding T cells. When contrasted with standard mixed lymphocyte cultures, addition of GVHD-derived CD4+CD25+ regulatory T cells yielded a 15–20 fold increase in 3H-thymidine incorporation. Thus, GVHD-associated CD4+CD25+ regulatory T cells have the unique ability to differentially regulate immune responses, promoting proliferation of naive alloresponsive T cells while suppressing proliferation of previously activated GVHD-derived alloreactive CD4+CD25− T cells. These data suggest that GVHD-derived CD4+CD25+ regulatory T cells may contribute to progression of GVHD in two ways: 1) By facilitating activation of allospecific naive T cells, providing a mechanism for further amplification of the immunoinflammatory cascade associated with this disease; and 2) By maintaining a reservoir of previously activated allospecific T cells, cells that may be available for supplementation of disease-associated immunoinflammatory responses as needed.