The impact of CD4+Foxp3+ Treg on immunity to murine cytomegalovirus after bone marrow transplantation depends on the peripheral or thymic source of T cell regeneration.

Objective: The adoptive transfer of regulatory T cells (Treg) in murine models has been shown to ameliorate graft-versus-host disease while it may preserve the graft-versus-leukemia effect. However, the impact of Treg on infectious immunity after bone marrow transplantation (BMT) is still unclear. Immunocompetence against opportunistic viral infections depends on the kinetics of T cell recovery after BMT through two distinctive processes, i.e. lymphopenia-induced proliferation (LIP) of mature T cells and generation of T cells through thymopoiesis.

Methods: In this study, we set out to assess the effects of adoptively transferred Treg on T cell regeneration in a homeostatic peripheral T cell expansion model and a thymopoiesis-dependent BMT model, and on murine cytomegalovirus (mCMV) clearance and mortality following mCMV challenge.

Results: Using lymphopenic Rag-2(-/-)γc(-/-) mice that received a limited number of congenic T cells, we demonstrate that adoptively transferred Treg abrogate LIP of T cells. mCMV challenge resulted in a rapid increase of viral load and death in mice that received Treg, but not in controls. In contrast, following syngeneic T cell-depleted BMT in Rag-2(-/-)γc(-/-) mice, adoptively transferred Treg did not delay T cell reconstitution nor suppressed thymic output and had no effect on viral clearance and survival following mCMV-challenge.

Conclusion: The effect of Treg on T cell-mediated immunocompetence against mCMV early after BMT depends on the relative contribution of peripheral expansion and thymopoiesis to T cell regeneration.