In-situ proliferation contributes to the accumulation of myeloid cells in the spleen during progressive experimental visceral leishmaniasis

Abstract

Visceral leishmaniasis (VL) is characterized by expansion of myeloid cells in the liver and spleen, which leads to a severe splenomegaly associated with higher risk of mortality. This increased cellularity is thought to be a consequence of recruitment of cells to the viscera. We studied whether the local proliferation of splenic myeloid cells contributes to increased splenic cellularity. We found that a monocyte-like population of adherent splenic cells from Leishmania donovani-infected hamsters had enhanced replicative capacity ex vivo and in vivo (BrdU incorporation, p<0.0001). In vitro assays demonstrated that proliferation was more pronounced in the proinflammatory M1 environment and that intracellular infection prevented proliferation. Secondary analysis of the published splenic transcriptome in the hamster model of progressive VL revealed a gene expression signature that included division of tumoral cells (Z = 2.0), cell cycle progression (Z = 2.3), hematopoiesis (Z = 2.8), proliferation of stem cells (Z = 2.5) and overexpression of proto-oncogenes. Regulators of myeloid cell proliferation were predicted in-silico (CSF2, TLR4, IFNG, IL-6, IL-4, RTK signaling, and STAT3). The in-silico prediction was confirmed with chemical inhibitors of PI3K/AKT, MAPK and STAT3 which decreased splenic myeloid cell division ex vivo. Hamsters infected with L. donovani treated with a STAT3 inhibitor had reduced in situ splenic myeloid proliferation (p = 0.03) and parasite burden. We conclude that monocyte-like myeloid cells have increased STAT3-dependent proliferation in the spleen of hamsters with visceral leishmaniasis and that inhibition of STAT3 reduces myeloid cell proliferation and parasite burden.

Persistent Cutaneous Leishmania major Infection Promotes Infection-Adapted Myelopoiesis

“…This accumulation was also accompanied by an increase in myeloid-biased progenitor cells, suggesting that some of these cells could indeed be the result of myeloid differentiation in the spleen. It has been shown in the hamster model of visceral leishmaniasis that at least some of the myeloid cells accumulating in the spleen could be the result of intrasplenic proliferation [ 58 ]; however, this does not exclude the potential contribution of enhanced myelopoiesis as immature cells are more likely to proliferate than mature myeloid cells. There was also an accumulation of platelets and an increase in overall spleen cellularity in mice infected with the non-healing Lm Sd strain, further suggesting that spleen could be a site of active hematopoiesis in persistent cutaneous leishmaniasis.…”

Determinants of Innate Immunity in Visceral Leishmaniasis and Their Implication in Vaccine Development

“…The iMOs recruited to the spleen during L. donovani infection can acquire a hypoxia‐inducible factor (HIF)-1α mediated myeloid-derived suppressor cells (MDSC) like phenotype, which promotes a chronic infection ( 80 ). L. donovani infection of hamsters has been shown to induce extramedullary hematopoiesis of myeloid cells in the spleen resulting in the accumulation of monocytes that support parasite replication ( 81 ). Migration of Ly6C hi iMOs to the spleen and liver is facilitated by STAT-1 signaling ( 59 , 82 ).…”

Effects of Visceralising Leishmania on the Spleen, Liver, and Bone Marrow: A Pathophysiological Perspective

“…The inflammatory state that is elicited, either by Leishmania or by other pathogens, drives the release of immature myeloid lineage from the BM [ 49 , 52 ]. These cells, as we explain later, are conditioned by the BM microenvironment and/or by their infection with Leishmania to become myeloid-derived suppressor cells (MDSC) in the spleen [ 52 , 53 ].…”

“…The inflammatory state that is elicited, either by Leishmania or by other pathogens, drives the release of immature myeloid lineage from the BM [ 49 , 52 ]. These cells, as we explain later, are conditioned by the BM microenvironment and/or by their infection with Leishmania to become myeloid-derived suppressor cells (MDSC) in the spleen [ 52 , 53 ]. They induce a strong anti-inflammatory response, with increased amounts of IL-10 and TGF-β, further dampening an effective immune response and perpetuating the infection [ 7 , 49 , 52 , 53 , 54 , 55 ].…”

“…These cells, as we explain later, are conditioned by the BM microenvironment and/or by their infection with Leishmania to become myeloid-derived suppressor cells (MDSC) in the spleen [ 52 , 53 ]. They induce a strong anti-inflammatory response, with increased amounts of IL-10 and TGF-β, further dampening an effective immune response and perpetuating the infection [ 7 , 49 , 52 , 53 , 54 , 55 ]. DCs residing in the disorganized, inflammatory, and markedly hypoxic splenic environment show increased HIF1 stabilization, which hampers their ability to produce the proinflammatory cytokine IL-12 and diverts them further toward IL-10 secretion [ 56 , 57 , 58 , 59 ].…”