High Proportions of CD4⁺CD25^hi T Cells Pretransplant Correlate with Worse Overall Survival after Stem Cell Transplant.

Regulatory cells may have a modulatory effect on alloreactive phenomena following hematopoietic stem cell transplantation (HSCT), although their role in this setting in patients remains controversial. We have analysed the effect of pre-conditioning peripheral blood levels of T-regs on the outcome of 89 adult patients undergoing HLA-identical (10/10) unrelated donor (UD)-HSCT. Allografts were T-cell depleted with Alemtuzumab, which has been described to spare T-regs from, and we found that higher proportion of CD4+CD25hi T-cells correlated with worse survival (p=0.002). This higher proportion also correlated with higher incidence of relapse (p=0.0274) and with higher incidence of chronic (c) GvHD (p=0.033) but not with the incidence of aGvHD.

Regulatory T cells (T-regs) are a naturally occurring regulatory population of CD4⁺CD25^hi T-cells known to express the transcription repressor FoxP3 and to produce anti-inflammatory cytokines such as TGFβ and may therefore affect patient survival. We analyse the profile of the CD4⁺CD25^hiT cells in our cohort. In our healthy donor control group (n=30), CD4⁺CD25^hi-expression associated with both, FoxP3 mRNA-levels (r=0.649; p=0.001) and TGFβ production (r=0.912; p<0.001), as previously described for T-regs. In preconditioning patient samples however, CD4+CD25hi-expression had a strong correlation with TGFβ regulatory cytokine production (r=0.863; p<0.001) as expected. However, FoxP3 mRNA-expression correlated neither with the CD4+CD25hi T-regs phenotype (r=0.280; p=0.040), nor with the production of TGFβ (r=0.229; p=0.156), and appeared not be an accurate marker for regulatory T-cell function in this patient setting. In addition, an inverse correlation with TNFα expression (r=−0.458 p<0.001) was found that may argue that the CD4⁺CD25^hi cells represent a true regulatory population and not activated cells. This suggests either that activated cells that are not expressing FOXP3 might be included in the CD4⁺CD25^hi population or that CD4⁺CD25^loFOXP3⁺ cells have not acquire the regulatory function (i.e. they are not expressing TGFβ).

In summary, CD4⁺CD25^hi Regulatory T cells may have an impact suppressing allo-responses against the tumour, decreasing the overall survival by increasing the rates of relapse.

Although, in mice CD4⁺CD25^hi population have been clearly identified by the expression of the FOXP3, which encodes a transcription repressor, in humans this remains controversial, where FOXP3 expression is not exclusive of the CD4⁺CD25^hi population.

Relying on the expression of CD4⁺CD25^hi and FOXP3 in patients is insufficient to determine regulatory T cell numbers and suggests that other parameters of regulatory function should be taken in to account.