Splenic CD4+ T Cells in Progressive Visceral Leishmaniasis Show a Mixed Effector-Regulatory Phenotype and Impair Macrophage Effector Function through Inhibitory Receptor Expression

Abstract

Visceral leishmaniasis (VL), caused by infection with the intracellular protozoan Leishmania donovani, is a chronic progressive disease with a relentlessly increasing parasite burden in the spleen, liver and bone marrow. The disease is characterized by fever, splenomegaly, cachexia, and pancytopenia, and progresses to death if not treated. Control of Leishmania infection is mediated by Th1 (IFNγ-producing) CD4+ T cells, which activate macrophages to produce nitric oxide and kill intracellular parasites. However, despite expansion of CD4+ T cells and increased IFNγ expression in the spleen, humans with active VL do not control the infection. We used an experimental model of chronic progressive VL in hamsters, which mimics clinical and pathological features seen in humans, to better understand the mechanisms that lead to progressive disease. Transcriptional profiling of the spleen during chronic infection revealed expression of markers of both T cell activation and inhibition. CD4+ T cells isolated from the spleen during chronic progressive VL showed mixed expression of Th1 and Th2 cytokines and chemokines, and were marginally effective in controlling infection in an ex vivo T cell-macrophage co-culture system. Splenic CD4+ T cells and macrophages from hamsters with VL showed increased expression of inhibitory receptors and their ligands, respectively. Blockade of the inhibitory receptor PD-L2 led to a significant decrease in parasite burden, revealing a pathogenic role for the PD-1 pathway in chronic VL. PD-L2 blockade was associated with a dramatic reduction in expression of host arginase 1, but no change in IFNγ and inducible nitric oxide synthase. Thus, the expression of counter-regulatory molecules on splenic CD4+ T cells and macrophages promotes a more permissive macrophage phenotype and attenuates intracellular parasite control in chronic progressive VL. Host-directed adjunctive therapy targeting the PD-1 regulatory pathway may be efficacious for VL.

The Effect of Iodium 30c on Experimental Visceral Leishmaniasis

“…It has also been observed that during sub-clinical L. donovani infection, enhanced percentages of CD4 þ T cells resulted in disease control. 38 In the present study, a significant decline in the CD4 þ T cell population was observed in the infected animals compared with the normal controls, whilst the treatment of animals with Iodium 30c led to significant reversal of the suppressed immune response. This is evident also in patients with active VL, where a higher population of CD8 þ T cells than CD4 þ T cells has been observed, which reverted to normal levels following subsequent treatment.…”

Transcriptional Profiling of Immune-Related Genes in Leishmania infantum-Infected Mice: Identification of Potential Biomarkers of Infection and Progression of Disease

“…This approach of using differential transcriptome analysis as a tool to understand Leishmania-host interactions has been successfully employed in several animal models and in vitro studies. Despite differences related to experimental models (Syrian hamster/mice/ in vitro macrophages) and methodology (RNA-seq/qPCR/microarray hybridization) all of them draw a picture of mixed responses during infection and deactivation of effective parasite-controlling responses (Rabhi et al, 2012 ; Dillon et al, 2015 ; Fernandes et al, 2016 ; Kong et al, 2017 ; Medina-Colorado et al, 2017 ). Similar to our study, transcriptional profile of spleen samples from L. donovani -infected hamsters was analyzed 28 days after infection, revealing a strikingly proinflammatory environment and a strong expression of Ifng that did not protect against the increasing parasite burden (Kong et al, 2017 ).…”

“…Similar to our study, transcriptional profile of spleen samples from L. donovani -infected hamsters was analyzed 28 days after infection, revealing a strikingly proinflammatory environment and a strong expression of Ifng that did not protect against the increasing parasite burden (Kong et al, 2017 ). Likewise, chronic infection in hamsters revealed expression of markers of both T cell activation and inhibition, showing mixed expression of Th1 and Th2 cytokines and chemokines, and again ineffective in controlling infection (Medina-Colorado et al, 2017 ). Those studies that focused on the evaluation of early stages of infection (Dillon et al, 2015 ; Fernandes et al, 2016 ) revealed upregulation of both pro- and anti-inflammatory related genes similarly to our findings.…”

“…One of the major hurdles for developing vaccines to either prevent or treat VL has been a limited understanding of the precise immune mechanisms required for controlling parasite growth without causing disease. Because of the intrusive techniques required to analyze tissue in VL patients, our current understanding of the host immune response during VL largely derives from studies performed in genetically susceptible mice or hamsters infected with viscerotropic species (Faleiro et al, 2014 ; Kong et al, 2017 ; Medina-Colorado et al, 2017 ). These animals develop distinct, organ specific immune responses as disease progresses (Engwerda and Kaye, 2000 ; Rodrigues et al, 2016 ).…”

Tissue and host species-specific transcriptional changes in models of experimental visceral leishmaniasis

“…One study has examined whole blood transcriptional responses in healthy controls versus asymptomatic and symptomatic VL patients in Brazil 39 . Three studies have evaluated transcriptional changes in the spleen of hamsters infected with L. donovani 40 – 42 . Here, we have applied transcriptional profiling to address key aspects of the tissue specific response to L. donovani infection in mice.…”