Pathogenic Leptospires Limit Dendritic Cell Activation Through Avoidance of TLR4 and TRIF Signaling

Abstract

Leptospira interrogans is a bacterial species responsible for leptospirosis, a neglected worldwide zoonosis. Mice and rats are resistant and can become asymptomatic carriers, whereas humans and some other mammals may develop severe forms of leptospirosis. Uncommon among spirochetes, leptospires contain lipopolysaccharide (LPS) in their outer membrane. LPS is highly immunogenic and forms the basis for a large number of serovars. Vaccination with inactivated leptospires elicits a protective immunity, restricted to serovars with related LPS. This protection that lasts in mice, is not long lasting in humans and requires annual boosts. Leptospires are stealth pathogens that evade the complement system and some pattern recognition receptors from the Toll-like (TLR) and Nod-Like families, therefore limiting antibacterial defense. In macrophages, leptospires totally escape recognition by human TLR4, and escape the TRIF arm of the mouse TLR4 pathway. However, very little is known about the recognition and processing of leptospires by dendritic cells (DCs), although they are crucial cells linking innate and adaptive immunity. Here we tested the activation of primary DCs derived from human monocytes (MO-DCs) and mouse bone marrow (BM-DCs) 24h after stimulation with saprophytic or different pathogenic virulent or avirulent L. interrogans. We measured by flow cytometry the expression of DC-SIGN, a lectin involved in T-cell activation, co-stimulation molecules and MHC-II markers, and pro- and anti-inflammatory cytokines by ELISA. We found that exposure to leptospires, live or heat-killed, activated dendritic cells. However, pathogenic L. interrogans, especially from the Icterohaemorraghiae Verdun strain, triggered less marker upregulation and less cytokine production than the saprophytic Leptospira biflexa. In addition, we showed a better activation with avirulent leptospires, when compared to the virulent parental strains in murine BM-DCs. We did not observe this difference in human MO-DCs, suggesting a role for TLR4 in DC stimulation. Accordingly, using BM-DCs from transgenic deficient mice, we showed that virulent Icterohaemorraghiae and Manilae serovars dampened DC activation, at least partly, through the TLR4 and TRIF pathways. This work shows a novel bacterial immune evasion mechanism to limit DC activation and further illustrates the role of the leptospiral LPS as a virulence factor.

Impaired functions of human monocyte-derived dendritic cells and induction of regulatory T cells by pathogenic Leptospira

“…Our findings reveal that pathogenic leptospires partially impair MoDC maturation and skew MoDCs-CD4 + T cell interactions toward Treg cells, thereby failing to stimulate CD4 + T cell proliferation. In line with previous work [ 14 ], MoDCs-P exhibited lower expression levels of CD83 and CD80 compared to MoDCs-NP. Although CD86 and HLA-DR did not show significant differences, the overall expression profile of surface markers suggests impaired maturation of MoDCs-P relative to MoDCs-NP.…”

“…Despite this limitation, we observed higher levels of IL-10 and IL-12p70 secretion in MoDCs-NP compared to MoDCs-P. These findings align with previous studies that have demonstrated greater IL-10 production in MoDCs infected with saprophytic leptospires (serovar Patoc) than those infected with the pathogenic leptospires strain Verdun [14]. Furthermore, MoDCs-NP exhibited higher levels of IL-12p70 production than MoDCs-P, which may potentially indicate an induction toward Th1 cell responses.…”

“…Moreover, a pathogenic strain of leptospires demonstrated reduced activation of human MoDCs compared to non-pathogenic leptospires, as indicated by the lower percentage of CD83 + cells. Notably, pathogenic leptospires induced a lower production of both pro- and anti-inflammatory cytokines than the non-pathogenic strain [ 14 ]. However, it is worth noting that these results were not consistent for all the pathogenic strains examined in that study.…”

Cellular Pathophysiology of Leptospirosis: Role of Na/K-ATPase

“…In addition, critical antimicrobial responses and the O antigen appear to contribute to this failure [ 40 ]. Another consequence of impaired TLR4–TRIF signaling is reduced CD40 expression in dendritic cells (DC), a vital component for generating memory B cells [ 41 ]. The leptospiral LLS do not seem to have pyrogenic potential and have not been proved cytotoxic [ 31 ].…”

Comparative transcriptome analyses of immune responses to LPS in peripheral blood mononuclear cells from the giant panda, human, mouse, and monkey

“…However, TLR4 but not TLR2 plays a critical role in LPS signaling (Takeuchi et al, 1999), and LPS recognition is predominantly mediated by TLR4 (Poltorak et al, 1998). TLR4 belongs to a pattern recognition receptor (PRR), and it mainly expresses in macrophages, dendritic cells, eosinophilic granulocytes, and mastocytes (Cagliero et al, 2022;Jing et al, 2022). LPS can be recognized by TLR4 and activates the signaling pathway related to nuclear factor-kappa B (NF-κB) to promote the expression of cytokine genes and the release of cytokines in immune cells (Khan et al, 2022;Martino et al, 2022).…”