To the Editor: We read with interest the article by Rodriguez et al. in which an association between delayed-type-hypersensitivity (DTH)-regulation and HLA-DR matching was found (1Rodriguez DS Jankowska-Gan E Haynes LD et al.Immune regulation and graft survival in kidney transplant recipients are both enhanced by human leukocyte antigen matching.Am J Transplant. 2004; 4: 537-543Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Our unpublished data, presented at ICTS 2002, looking at regulation by human CD4+CD25+ cells in vitro was consistent with their findings, but with an important caveat (Game and Lechler, abstract O239). We have hypothesised that CD4+CD25+ cells are self-HLA restricted and specific for a variety of self-antigens, although cross-reactivity with alloantigens will also occur (2Game DS Hernandez-Fuentes MP Chaudhry AN Lechler RI CD4+CD25+ regulatory T cells do not significantly contribute to direct pathway hyporesponsiveness in stable renal transplant patients.J Am Soc Nephrol. 2003; 14: 1652-1661Crossref PubMed Scopus (66) Google Scholar). If this is the case then suppression of alloresponses is likely to be strongest in cases of HLA-DR matching. We performed mixed lymphocyte reactions using cells purified from volunteers of known HLA-DR type. The responder populations were either whole CD4+ populations or CD4+CD25− cells—we predicted that the increase in cell proliferation or IL2 secretion upon depletion of CD4+CD25+ cells reflected the degree of suppression mediated by these cells. By dividing the result after depletion by the result before depletion, we calculated a ‘suppression ratio’. Indeed, the suppression ratio did follow the pattern 0 > 1 > 2 mismatches for HLA-DR as shown in Figure 1; the result for IL2 secretion was comparable (n = 8). However, closer analysis of the data raised an alternative possibility, which is difficult to rule out, namely, the degree of suppression is inversely correlated to the strength of the response. This is shown in Figure 2, which is accumulated data for the proliferation of CD4+ cells in the experiments described. Of note, Figure 1 by Rodriguez et al. appears consistent with this alternative. Rodriguez et al. did not explore the nature of the regulatory cells suppressing the DTH responses, but it seems reasonable to assume that all regulatory cells operate in a dose-dependent fashion. The concept that CD4+CD25+ cells suppress weaker reactions better than stronger reactions is not new (3Baecher-Allan C Viglietta V Hafler DA Inhibition of human CD4(+)CD25(+high) regulatory T cell function.J Immunol. 2002; 169: 6210-6217Crossref PubMed Scopus (227) Google Scholar) and must be taken into account when interpreting data on HLA-DR matching and regulation. In either case, the data presented by Rodriguez et al. are consistent with the notion that regulatory cells, being self-HLA restricted and/or better at regulating weaker alloresponses, will regulate the indirect pathway of allorecognition more efficiently than the direct pathway (2Game DS Hernandez-Fuentes MP Chaudhry AN Lechler RI CD4+CD25+ regulatory T cells do not significantly contribute to direct pathway hyporesponsiveness in stable renal transplant patients.J Am Soc Nephrol. 2003; 14: 1652-1661Crossref PubMed Scopus (66) Google Scholar, 4Salama AD Najafian N Clarkson MR Harmon WE Sayegh MH Regulatory CD25+ T cells in human kidney transplant recipients.J Am Soc Nephrol. 2003; 14: 1643-1651Crossref PubMed Scopus (205) Google Scholar).