Trypanosoma brucei brucei causes a rapid and persistent influx of neutrophils in the spleen of infected mice

Abstract Trypanosomosis is a chronic parasitic infection, affecting both humans and livestock. A common hallmark of experimental murine infections is the occurrence of inflammation and the associated remodelling of the spleen compartment. The latter involves the depletion of several lymphocyte populations, the induction of T‐cell‐mediated immune suppression, and the activation of monocyte/macrophage cell populations. Here, we show that in experimental T b brucei infections in mice, these changes are accompanied by the alteration of the spleen neutrophil compartment. Indeed, mature neutrophils are rapidly recruited to the spleen, and cell numbers remain elevated during the entire infection. Following the second peak of parasitemia, the neutrophil cell influx coincides with the rapid reduction of splenic marginal zone (MZ)B and follicular (Fo)B cells, as well as CD8+ T and NK1.1+ cells, the latter encompassing both natural killer (NK) and natural killer T (NKT) cells. This report is the first to show a comprehensive overview of all alterations in spleen cell populations, measured with short intervals throughout the entire course of an experimental T b brucei infection. These data provide new insights into the dynamic interlinked changes in spleen cell numbers associated with trypanosomosis‐associated immunopathology.

Neutrophils are known to play a key role in the first line of defence against invading pathogens via the innate arm of the immune system. Upon arrival at the site of inflammation, neutrophils engage their effector functions by eliminating invading pathogens and trigger inflammatory reactions. [14][15][16] However, recent data demonstrate that neutrophils can also extend their functions beyond their role in pathogen clearance and can play a role in promoting parasite survival, in particular, during the onset of tsetse-transmitted trypanosomosis. 17 The lack of systematic data on quantitative changes in spleen cell numbers throughout infection prompted the data collection reported here.
Parasitemia was assessed as previously described. 18

| Statistical analysis
Prism ® 7.0 software (GraphPad Software Inc) was used to graphically represent data and perform statistical analysis, using unpaired student t tests. Data are presented as mean ± SD.

| RE SULTS
Spleen leucocyte population changes were analysed during T b brucei AnTat1.1E infections. Table 1 shows the number of spleen, early TA B L E 1 Immune cell populations in the spleen of Trypanosoma brucei brucei infected C57BL/6 mice   Coinciding with the clearance of the first parasitemia peak (6 dpi), a 5-fold increase in spleen neutrophil cells is observed ( Figure 1D,

| D ISCUSS I ON
While analysing trypanosomosis-induced anaemia in the past, we

CO N FLI C T O F I NTE R E S T
The authors declare that the research was conducted in the absence of any commercial of financial relationships that could be construed as a potential conflict of interest.

AUTH O R CO NTR I B UTI O N S
Research design: CD MR SM. Research acquisition: VD HTTP ID IJ.
Data analysis and interpretation: VD ID HTTP IJ CD MR SM. Drafting of paper: VD. Revising of paper: CD MR SM.

DATA AVA I L A B I L I T Y
The data that support the findings of this study are available from the corresponding author upon reasonable request.