The monocytosis during human leptospirosis is associated with modest immune cell activation states.

Leptospirosis is a life-threatening zoonotic disease and it has been hypothesized that the innate immune system fails to control the infection through ill-characterized mechanisms. The aim of this observational study was to better evaluate the activation processes of monocytes at the early stage of the disease. Blood samples were taken from healthy donors (n = 37) and patients hospitalized for either non-severe (n = 25) or severe (n = 32) leptospirosis. Monocyte cell counts and phenotypes were assessed by flow cytometry. We analysed the expression of several cell activation markers: CD14, CD16, HLA-DR, CD69, TLR2, TLR4, CD11b and CD11c. Although monocyte values at admittance were not significantly different from controls, patients experienced significant monocytosis at 1.33 × 109 /L (p < 0.0001 compared to controls: 0.56 × 109 /L) during their hospital stay. This monocytosis observed during hospital stay was correlated to several surrogate markers of organ injury. Non-classical (CD14-CD16+) and intermediate (CD14+CD16+) monocyte subsets increased compared to controls (p < 0.05). Accordingly, classical monocyte subset (CD14+CD16-) showed decreased percentages (p < 0.0001). Levels of several cell surface activation molecules were decreased: HLA-DR involved in MHC class II antigen presentation, integrins CD11b and CD11c implicated in phagocytosis and cell recruitment (p < 0.0001). None of these parameters had a prognostic value. Results from this study showed that during acute human leptospirosis, patients experienced monocytosis with a switch toward an inflammation-related phenotype contrasted by low expression levels of markers implicated in monocyte function.