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“Faster, better, more” is the conventional benchmark used to define responses of memory T cells when compared with their naïve counterparts. In this issue of the European Journal of Immunology, Mark and Warren Shlomchik and colleagues [Eur. J. Immunol. 2011. 41: 2782–2792] make the intriguing observation that murine memory CD4+ T-cell populations enriched for alloreactive precursors

are fully capable of rejecting allogeneic skin grafts but yet are incapable of inducing significant ZD1839 in vitro graft-versus-host disease. These observations add to the emerging concept that memory CD4+ T-cell development is more nuanced and complex than predicted by conventional models. In particular, the data suggest that it may

be just as important to consider what naïve or effector cells have “lost” in their transition see more to memory. Memory T cells with reactivity against alloantigens are generally considered to constitute a major barrier to successful solid organ transplantation 1. Alloreactive memory CD4+ T-cell populations rapidly generate secondary effectors or provide help to B cells to promote the generation of alloantigen-specific antibody. These memory cells are resistant to both tolerance induction through costimulatory blockade 2 or immunosuppression by regulatory T cells 3. It might be expected therefore that transfer of such memory CD4+ T-cell populations to allogeneic bone marrow transplantation (BMT) recipients would lead to severe graft-versus-host disease (GVHD). The fact that GVHD does not occur when such experiments are performed, as reported in this issue of the European Journal of Immunology by Mark and Warren Protein tyrosine phosphatase Shlomchik and colleagues 4, suggests an unexpected level of heterogeneity and complexity

in the functions of memory CD4+ T cells. Transfer of donor T cells into recipients during allogeneic experimental BMT induces GVHD in a highly predictable manner 5. The allogeneic T-cell response occurs in the context of host injury induced by the conditioning treatments required prior to BMT, leading to severe inflammation and the rapid accumulation of T-cell effectors in peripheral tissue such as the gut, skin, and liver. Damage to the thymus 6 and the stroma of secondary lymphoid organs (SLOs) and BM 7 leads to a state of profound immunodeficiency, increasing the risk of infection. There has therefore been a strong clinical interest in developing strategies that permit effective immune reconstitution following BMT without induction of GVHD. This provided the incentive for a number of groups to explore the role of individual T-cell subsets in conferring GVHD.