Neutrophil metalloproteinase driven spleen damage hampers infection control of trypanosomiasis

Abstract

Recent blood transcriptomic analysis of rhodesiense sleeping sickness patients has revealed that neutrophil signature genes and activation markers constitute the top indicators of trypanosomiasis-associated inflammation. Here, we show that Trypanosoma brucei infection results in expansion and differentiation of four splenic neutrophil subpopulations, including Mki67+Birc5+Gfi1+Cebpe+ proliferation-competent precursors, two intermediate immature subpopulations and Cebpb+Spi1+Irf7+Mcl1+Csf3r+ inflammation reprogrammed mature neutrophils. Transcriptomic scRNA-seq profiling identified the largest immature subpopulation by Mmp8/9 positive tertiary granule markers. We confirmed the presence of both metalloproteinases in extracellular spleen homogenates and plasma. During infection, these enzymes digest extracellular matrix components in the absence of sufficient TIMP inhibitory activity, driving remodeling of the spleen follicular architecture. Neutrophil depletion prevents the occurrence of organ damage, resulting in increased plasma cell numbers and prolonged host survival. We conclude that trypanosomiasis-associated neutrophil activation is a major contributor to the destruction of the secondary lymphoid architecture, required for maintaining an efficient adaptive immune response.

Advances in the Immunology of the Host–Parasite Interactions in African Trypanosomosis, including Single-Cell Transcriptomics

“…In addition, infection-associated B cell depletion will ultimately reduce the efficacy of vaccines unrelated to trypanosomosis [ 12 , 118 ]. While in the past, events related to the infection-associated abrogation of B cell functionality have been mainly described in a phenotypic and functional cellular context, most recent data has now come from scRNA-seq approaches [ 107 , 116 , 119 ]. The obtained data will be discussed below in an organ-relevant context.…”

“…This finding was recently confirmed, showing a 15-fold increase in spleen neutrophil cell numbers by the 2nd week of infection. Hence, this infection time point was used to perform a detailed study of cellular neutrophil heterogeneity at the transcriptome level [ 119 ]. In line with the known neutrophil maturation process [ 136 , 137 , 138 ], four subpopulations (N1–N4) were also found in the T. b. brucei infection model.…”

“…Interestingly, no clear infection-induced neutrophil number alteration was observed at the bone marrow level. This suggests that neutrophil differentiation, which usually occurs in bone marrow, is triggered directly in the spleen during T. b. brucei infections [ 119 ]. This is interesting as neutrophils are considered to be cells that can play a double-edged sword role during infection, helping either in pathogen elimination or being responsible for tissue damage and organ pathology.…”

“…Hence, the recent results of both scRNA-seq analysis and actual protease concentration confirmation assay both have now shown that trypanosomes trigger excessive MMP-8 and MMP-9 release and activation, in particular, involving the N3 subpopulation, in the absence of TIMP inhibitory activity. This ultimately leads to the destruction of the ECM and damage to the splenic follicular architecture and once again aids the parasite by depriving the host of the capacity to generate an immune environment needed for producing high-affinity antibodies [ 119 ]. Indeed, trypanosomosis-associated B cell depletion coincides with the neutrophil-driven destruction of spleen germinal center architecture.…”

“…Surprisingly, this data showed (i) that the 16-fold cell number increase in the spleen did not occur as a result of bone marrow-spleen cell migration and (ii) that the spleen neutrophil increase is not mirrored in the bone marrow, where only a 1.3-fold cell increase was noticed. In addition, while spleen data had shown a marked effect on neutrophil differentiation as a result of ongoing trypanosome infection, the latter had no significant effect on the overall bone marrow neutrophil differentiation pattern [ 119 ]. However, various time point assessments and additional studies in other trypanosome species are needed to obtain a more comprehensive view of how trypanosome infections affect the bone marrow cellular composition and function.…”