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Plasmodium yoelii infection of BALB/c mice results in expansion rather than induction of CD4+ Foxp3+ regulatory T cells

Recently, we demonstrated elevated numbers of CD4+ Foxp3+ regulatory T (Treg) cells in Plasmodium yoelii‐infected mice contributing to the regulation of anti‐malarial immune response. However, it remains unclear whether this increase in Treg cells is due to thymus‐derived Treg cell expansion or induction of Treg cells in the periphery. Here, we show that the frequency of Foxp3+ Treg cells expressing neuropilin‐1 (Nrp‐1) decreased at early time‐points during P. yoelii infection, whereas percentages of Helios+ Foxp3+ Treg cells remained unchanged. Both Foxp3+ Nrp‐1+ and Foxp3+ Nrp‐1− Treg cells from P. yoelii‐infected mice exhibited a similar T‐cell receptor Vβ chain usage and methylation pattern in the Treg‐specific demethylation region within the foxp3 locus. Strikingly, we did not observe induction of Foxp3 expression in Foxp3− T cells adoptively transferred to P. yoelii‐infected mice. Hence, our results suggest that P. yoelii infection triggered expansion of naturally occurring Treg cells rather than de novo induction of Foxp3+ Treg cells.