LAG-3- and CXCR5-expressing CD4 T cells display progenitor-like properties during chronic visceral leishmaniasis
In chronic infections, sustaining the CD4 T cell responses is crucial for pathogen clearance.  The expression of inhibitory receptors such as LAG-3 or Lymphocyte activation gene 3 is usually associated with immune suppression and T cell exhaustion. However, LAG-3 also serves as an immune checkpoint molecule and is implicated in proliferation control and cell survival. In this study, we have utilized a mouse model of visceral leishmaniasis (VL), to characterize a subset of LAG-3- and CXCR5-expressing CD4 T cells. These LAG-3+CXCR5+PD-1lo/int CD4 T cells exist in naïve mice, they expand during VL, and are antigen specific. Importantly, the RNAseq analysis showed that these cells highly express gene signatures associated with self-renewal capacity, indicating that these cells might act as progenitors. Additionally, upon adoptive transfer into Rag1-/- mice followed by challenge with Leishmania donovani, these LAG-3+CXCR5+ PD-1lo/int CD4 T cells gave rise to different types of T effector and regulatory subsets, demonstrating their differentiation potential. These cells were also observed in mice infected with LCMV clone-13 and Heligmosomoides polygyrus bakeri. In summary, we propose that these LAG-3+CXCR5+ PD-1lo/int CD4 T cells that exhibit progenitor-like capacities could be implicated in maintaining CD4 T cell responses in chronic infections. Overall design: LAG-3+ CXCR5+ PD-1lo/int CD4 T cells (referred to as TLCPlo/int cells in the associated article) and CXCR5+ PD-1hi Tfh cells (mostly germinal center Tfh cells) were purified by cell sorting from spleens of Leishmania donovani-infected mice (n = 6) at 21 days post infection. We performed bulk-RNA-seq to compare TLCPlo/int cells with GC-Tfh cells, as the initial flow cytometry analysis showed that these two populations shared similar markers such as CXCR5, PD-1, ICOS and TCF-1.