The role of Toll-like receptors and C-type lectins for vaccination against Candida albicans

Recent progress has provided important novel insights in the processes driving the adaptive immune responses. Central to these developments is the discovery of pattern recognition receptors like TLRs and CLRs that not only induce innate immune responses, but also modulate cellular and humoral adaptive immunity. As vaccination is one of the great achievements in medicine and probably the most powerful tool to protect human and animals against infectious disease, further vaccine development and optimization of current strategies can improve health status of large groups of people. Development of a vaccine against Candida spp. should induce both cellular and humoral immune responses. While the TLRs are strong inducers of inflammatory responses, it seems that the CLRs have the potential to modulate these responses by enhancement or inhibition of cytokine production. Understanding the natural host defense mechanisms against pathogens like C. albicans therefore helps to identify the proper targets for inducing a strong adjuvant effect, in order to stimulate an effective adaptive immune response and protection.     

Immune cells fold and damage fungal hyphae

Innate immunity provides essential protection against life-threatening fungal infections. However, the outcomes of individual skirmishes between immune cells and fungal pathogens are not a foregone conclusion because some pathogens have evolved mechanisms to evade phagocytic recognition, engulfment, and killing. For example, Candida albicans can escape phagocytosis by activating cellular morphogenesis to form lengthy hyphae that are challenging to engulf. Through live imaging of C. albicans–macrophage interactions, we discovered that macrophages can counteract this by folding fungal hyphae. The folding of fungal hyphae is promoted by Dectin-1, β2-integrin, VASP, actin–myosin polymerization, and cell motility. Folding facilitates the complete engulfment of long hyphae in some cases and it inhibits hyphal growth, presumably tipping the balance toward successful fungal clearance.

An estimated 1.5 million people succumb to a systemic fungal infection each year (1). Most of these individuals had HIV or had undergone a medical intervention that severely compromised their immunity. The innate immune system plays a key role in preventing fungal infection (2–4). The efficacy of these defenses depends on the outcome of individual interactions between innate immune cells and fungal pathogens.Innate immune cells such as macrophages recognize fungal pathogens via pattern recognition receptors (PRRs) that interact with pathogen-associated molecular patterns (PAMPs), many of which lie at the fungal cell surface (5, 6). The formation of a phagocytic synapse between PRRs and PAMPs triggers the active engulfment of the pathogen and subsequent attempts to kill the fungal cell using a variety of mechanisms that include a toxic mix of reactive chemical species and antimicrobial peptides (2). Meanwhile, fungal pathogens attempt to evade immune recognition, phagocytosis, and killing through a range of strategies that include PAMP masking to reduce recognition (7, 8), robust stress responses to attenuate the potency of reactive oxygen and nitrogen species (9–11), the activation of pyroptosis to kill the immune cells (12–14), and in particular, cellular morphogenesis (15–18). Candida albicans activates morphogenetic programs to form hyphae that are challenging to phagocytose and clear, not least because of their extreme length (6, 15, 16). Hypha formation also provides a means of fungal escape from the macrophage (12, 13).While examining dynamic interactions between macrophages and fungal cells, we observed that these immune cells can fold fungal hyphae. We reasoned that this fungal folding must involve the application of mechanical forces and that this folding contributes to fungal clearance. Therefore, we examined the involvement of the cytoskeletal network and PRRs in this phenomenon, providing initial clues as to how macrophages anchor a fungal hypha and achieve leverage to fold it.     

CX3CR1 Is Dispensable for Control of Mucosal Candida albicans Infections in Mice and Humans

Candida albicans is part of the normal commensal microbiota of mucosal surfaces in a large percentage of the human population. However, perturbations of the host''s immune response or bacterial microbiota have been shown to predispose individuals to the development of opportunistic Candida infections. It was recently discovered that a defect in the chemokine receptor CX₃CR1 increases susceptibility of mice and humans to systemic candidiasis. However, whether CX₃CR1 confers protection against mucosal C. albicans infection has not been investigated. Using two different mouse models, we found that Cx3cr1 is dispensable for the induction of interleukin 17A (IL-17A), IL-22, and IL-23 in the tongue after infection, as well as for the clearance of mucosal candidiasis from the tongue or lower gastrointestinal (GI) tract colonization. Furthermore, the dysfunctional human CX₃CR1 allele CX₃CR1-M280 was not associated with development of recurrent vulvovaginal candidiasis (RVVC) in women. Taken together, these data indicate that CX₃CR1 is not essential for protection of the host against mucosal candidiasis, underscoring the dependence on different mammalian immune factors for control of mucosal versus systemic Candida infections.     

Chronic yersiniosis due to defects in the TLR5 and NOD2 recognition pathways

Infection with Yersinia enterocolitica leads to a self-limiting disease, but in a small number of cases a protracted course can develop. The host genetic factors contributing to the advancement of the disease to the chronic phase are not known. We describe a patient suffering from an abdominal inflammatory mass due to chronic yersiniosis. Functional assays revealed defects in the recognition of flagellin by Toll-like receptor 5 (TLR5) and of muramyl dipeptide by NOD2, leading to a defective inflammatory response to Yersinia enterocolitica. Genetic sequencing showed that the patient was compound heterozygous for five different mutations in TLR5, while being homozygous for the 3020insC NOD2 mutation. In conclusion, we describe a patient in whom specific defects in the TLR5 and NOD2 recognition pathways led to chronic yersiniosis.     

Impaired dendritic cell function in Crohn's disease patients with NOD2 3020insC mutation

The nucleotide oligomerization domain 2 (NOD2) 3020insC (NOD2fs) mutation increases susceptibility to Crohn's disease (CD), but the mechanism remains controversial. Loss-of-function and gain-of-function phenotypes have been described as a result of NOD2fs. Here, we show that dendritic cells (DC) derived from CD patients homozygous for this mutation respond normally to purified Toll-like receptor (TLR) ligands but fail to up-regulate the costimulatory molecules CD80 and CD86 in response to the NOD2 ligand muramyl dipeptide (MDP). Moreover, they lack MDP-induced enhancement of TLR-mediated tumor necrosis factor alpha, interleukin (IL)-12, and IL-10 production, which is observed in control DC with intact NOD2. These data indicate that the NOD2fs mutation results in a loss-of-function phenotype in human myeloid DC and imply decreased immune regulation by IL-10 as a possible mechanism for this mutation in CD.