Arginase 1 causes chronicity of Leishmania mexicana infection via the generation of a replicative monocytic host cell niche

Abstract text

Background and objective

Control of Leishmania (L.) parasites require interferon (IFN)γ-dependent type 2 nitric oxide synthase (NOS2), an enzyme that converts L-arginine into citrulline and nitric oxide (NO). NOS2 activity is counteracted by arginase (ARG) 1 and 2, both of which are induced by Th2 cytokines and cleave L-arginine into urea and ornithine, a precursor of polyamines. Recently, we observed that the expression of ARG1 and ARG2 steadily increased in L. mexicana-infected BALB/c and C57BL/6 mice during disease manifestation. Here, we studied the role of host cell arginases during L. mexicana-induced chronic cutaneous leishmaniasis (CL).

Methods

C57BL/6 wild-type (WT), germ-line k.o. (Arg2-/-) or cell-specific conditional k.o. mice (Arg1ΔTek, Arg1ΔCx3cr1, Il10ΔCd4) were infected into the skin with 3×106 L. mexicana promastigotes. Infected skin tissue was processed for mRNA/protein expression, metabolomics and single-cell (sc) RNAseq analyses at different time-points post infection (p.i.).

Results

C57BL/6 WT mice developed non-healing chronic CL, whereas Arg2-/- mice showed progressive disease with a delayed onset. Deletion of Arg1 in hematopoietic and endothelial cells (Arg1ΔTek) or in monocytes and macrophages (Arg1ΔCx3cr1) exhibited a strongly reduced pathology and ultimately resolved their skin lesions despite parasite persistence, suggesting that myeloid Arg1 accounts for chronic CL. Mice lacking IL-10 in CD4+ T cells (Il10ΔCd4) defined IL-10 as a factor inducing ARG1 during infection. Metabolomics revealed the depletion of L-arginine by ARG1 and a significant rise in polyamines in the infected WT skin and draining lymph nodes. At day 40 p.i., when Arg1 mRNA was already upregulated in WT mice, but disease manifestation, parasite burden and the metabolic profile were still comparable between WT and Arg1-deficient mice, similar levels of NO were seen in WT and Arg1-deficient mice, although the expression of NOS2 protein was much higher in WT mice. ScRNAseq analysis of skin lesion cells identified distinct myeloid subpopulations that were enriched in WT mice, including a prominent Arg1+/NOS2+ and IFNγ-dependent cluster of inflammatory macrophages.

Conclusion

ARG1 serves as an unexpected signal for the recruitment and differentiation of monocytes in the skin of L. mexicana-infected mice, which leads to an enhanced immunopathology and the generation of a cellular niche that favors parasite replication due to ARG1/NOS2 substrate competition.