Human epithelial cells establish direct antifungal defense through TLR4-mediated signaling

Abstract

Mammalian TLRs are central mediators of the innate immune system that instruct cells of the innate and adaptive response to clear microbial infections. Here, we demonstrate that human epithelial TLR4 directly protected the oral mucosa from fungal infection via a process mediated by polymorphonuclear leukocytes (PMNs). In an in vitro epithelial model of oral candidiasis, the fungal pathogen Candida albicans induced a chemoattractive and proinflammatory cytokine response but failed to directly modulate the expression of genes encoding TLRs. However, the addition of PMNs to the C. albicans-infected model strongly upregulated cytoplasmic and cellsurface epithelial TLR4 expression, which correlated directly with protection against fungal invasion and cell injury. C. albicans invasion and cell injury was restored by the addition of TLR4-specific neutralizing antibodies and knockdown of TLR4 using RNA interference, even in the presence of PMNs, demonstrating the direct role of epithelial TLR4 in the protective process. Furthermore, treatment with neutralizing antibodies specific for TNF-α resulted in strongly reduced TLR4 expression accompanied by augmented epithelial cell damage and fungal invasion. To our knowledge, this is the first description of such a PMN-dependent, TLR4-mediated protective mechanism at epithelial surfaces, which may provide significant insights into how microbial infections are managed and controlled in the oral mucosa.

The role of pattern recognition receptors in the innate recognition ofCandida albicans

“…72 Nevertheless, the activation of human epithelial cell immune response is not likely to be initiated by TLRs, as blocking of these receptors with antibodies didn't alter the epithelial cytokine profile. 73 Dectin-1 and Dectin-2 are 2 confirmed receptors for C. albicans recognition. 74 However, only Dectin-1 is found expressed in epithelial cells.…”

“…81 C. albicans stimulates epithelial cells to produce IL-1a/b, IL-6, G-CSF, GM-CSF, TNF-a, and IL-8, while IL-12, IFN-g, IL-4, and IL-13 are not included. 73,[81][82][83][84] For example, IL-8 was released at high level by C. albicans-infected human oral epithelial cells and it actively recruits neutrophils into mucosal tissues, leading to an anti-fungal defense. 73 In primary human keratinocytes, IL-22 plus TNF-a effectively inhibit the growth of C. albicans and maintain the survival of epithelia.…”

“…73,[81][82][83][84] For example, IL-8 was released at high level by C. albicans-infected human oral epithelial cells and it actively recruits neutrophils into mucosal tissues, leading to an anti-fungal defense. 73 In primary human keratinocytes, IL-22 plus TNF-a effectively inhibit the growth of C. albicans and maintain the survival of epithelia. 85 In addition, human vaginal epithelial cells could express S100 calcium-binding proteins to recruit polymorphonuclear neutrophils (PMNs) to the C. albicans-infected vagina.…”

Candida albicansdimorphism as a therapeutic target

“…An important finding was that the hyphamediated MAPK/MKP1/c-Fos response was strongly dose-dependent, indicating that a threshold level of activation needs to be reached before epithelial cells are fully activated. The resulting proinflammatory cytokine response appears to recruit and activate polymorphonuclear cells, which subsequently protect the oral mucosa against C. albicans infection via a mechanism that requires Toll-like receptor (TLR)4 [59]. In summary, this epithelial MAPK-based activation pathway may comprise a 'danger response' mechanism, which may permit mucosal tissues to remain quiescent in the presence of low fungal burdens whilst responding specifically and strongly to damage-inducing hyphae when burdens increase.…”

Resposta imune inata na estomatite protética: interação in vitro entre células epiteliais de palato humano e Candida albicans

“…A interação in vitro fungo-epitélio passa por fases como aderência, invasão e dano celular (WEINDL et al, 2007;ZAKIKHANY et al, 2007), com a ativação dos mecanismos apoptóticos celulares por C. albicans auxiliando na promoção da patogênese fúngica (VILLAR; ZHAO, 2010;WAGENER et al, 2012).…”