Major Neutrophilia Observed in Acute Phase of Human Leptospirosis Is Not Associated with Increased Expression of Granulocyte Cell Activation Markers

Abstract

It has long been known that pathogenic Leptospira can mobilize the immune system but the specific contribution of neutrophils to control the infectious challenge remains to be clarified. We herein analyzed the phenotype of circulating neutrophils of patients with leptospirosis and healthy controls for the expression of toll-like receptor (TLR) type 2 (TLR2, to sense the leptospiral LPS) and several activation markers: interleukin 8 chemokine receptor CD182 (CXCR2), CD11b of the integrin/opsonin complement receptor type 3 (CR3) and CD15 (ligand of the selectin). The plasmatic level of the main CD182 ligand, interleukin 8 (CXCL8), was measured by ELISA. Hospitalized leptospirosis cases showed marked neutrophilia, particularly in the most severe cases. Interestingly, TLR2 was significantly increased in leptospirosis but identical levels of CD182 and CD11b were detected when compared to controls. CD15 was significantly decreased on neutrophils in leptospirosis but returned to normal within 1 month. Basal levels of IL-8 were measured in control subjects and were not increased in leptospirosis cases at the initial stage of the disease. In conclusion, we observed that neutrophils failed to regulate the expression of several of the receptors involved in cell activation and recruitment. This study further emphasizes the paradigm that neutrophils may be impaired in their overall capacity to thwart bacterial infection in leptospirosis patients.

Leptospiraspecies promote a pro‐inflammatory phenotype in human neutrophils

“…Interestingly, this neutrophilia correlated with the increased expression of CD15 (Raffray et al, ), a fact that supports our observation of enhanced chemotactic mechanisms, considering the role of this molecule in neutrophil chemotaxis (Kerr & Stocks, ). However, the same study reported no differences in CD11b surface levels on PMNs or in circulating levels of IL‐8 between controls and leptospirosis cases (Raffray et al, ). The discrepancies between our in vitro studies may be explained by several factors, including (a) we did not use CD16 to counterstain CD11b; (b) the relatively low number of studied cases (15), an important issue considering the tendency toward higher values of neutrophil activation markers shown in the previous study; and (c) the number of circulating bacteria was below 1 MOI; in most of our data, the Leptospira ‐mediated neutrophil effector responses were observed at higher MOI values.…”

“…Among the few previous studies of neutrophils in human leptospirosis, a significant increase in circulating PMNs has been reported during the acute phase of the disease (Lindow et al, ; Raffray et al, ). Interestingly, this neutrophilia correlated with the increased expression of CD15 (Raffray et al, ), a fact that supports our observation of enhanced chemotactic mechanisms, considering the role of this molecule in neutrophil chemotaxis (Kerr & Stocks, ). However, the same study reported no differences in CD11b surface levels on PMNs or in circulating levels of IL‐8 between controls and leptospirosis cases (Raffray et al, ).…”

“…promoted the formation of mixed platelet–neutrophil aggregates. Thrombocytopenia is a common feature of leptospirosis, but its origin is still not clear (Adiga, Mittal, Venugopal, & Mittal, ; de Silva et al, ; Goeijenbier et al, ; Raffray et al, ; Tunjungputri et al, ). In this regard, the formation of circulating mixed aggregates could represent a possible mechanism that accounts for the clearance of platelets and the consequent thrombocytopenia.…”

“…IL-6 is upregulated not only in response to a variety of signals, including bacterial components, but also by IL-1β and other cytokines, suggesting a complex activation network (Moon et al, 2000;Tuyt et al, 1999). Moreover, acting synergistically, they could also be responsible for the neutrophilia (Chiba et al, 2017) observed during acute leptospirosis in humans (de Silva et al, 2014;Lindow et al, 2016;Raffray et al, 2016).…”

“…Together, the findings in human and murine studies suggest that neutrophils do not seem to be an efficient defence factor for pathogenic Leptospira (Werts, ). On the other hand, it is well known that human leptospirosis presents neutrophilia, which correlates with severity (de Silva et al, ; Lindow et al, ; Raffray et al, ) and the pathogenic serovar involved (Craig et al, ).…”