Pattern-recognition receptors of the innate immune system bind components of microbes and initiate intracellular signal cascades that result in the activation of transcription factors, up-regulation of defense-associated target genes, and release of cytokines. Dectin-1 is a transmembrane pattern-recognition receptor that senses the β
-glucan component of fungal cell walls.23,41–43
On ligand binding, dectin-1 sends signals through an immunoreceptor tyrosine-based activation motif (ITAM), which becomes phosphorylated by Src family kinases (proto-oncogenic tyrosine kinases), leading to the recruitment and activation of the spleen tyrosine kinase (Syk).44,45
Dectin-1–Syk engages CARD9, which together with B-cell leukemia–lymphoma 10 (BCL10) and mucosa-associated lymphoid tissue 1 (MALT1) forms an intracellular signaling complex that in cells recognizing fungi leads to the activation of the transcription factor nuclear factor κ
B and mitogen-activated protein kinases.22,46–48
This signaling pathway is operative in myeloid cells and promotes the production of key cytokines, including interleukin-1β
, interleukin-6, and interleukin-23, which are required to control antifungal immune responses.24,38,49–52
Apart from dectin-1, the C-type lectins dectin-2 and macrophage-inducible C-type lectin (MINCLE) may also recognize fungi, engage the ITAM adapter FcRγ
for Syk activation, and transmit signals through the CARD9 pathway. 23,53–55
Thus, CARD9 plays a central role in antifungal defense by receiving signals from several antifungal pattern-recognition receptors and stimulating proinflammatory responses. Since murine Card9
deficiency results in susceptibility to fungal infections,22,39
this signaling pathway seems to be conserved between mice and humans.
Our study shows that a homozygous point mutation in CARD9, resulting in a premature termination codon and a loss of function in the adapter protein CARD9, is associated with a susceptibility to fungal infections, as evidenced by a chronic mucocutaneous candidiasis phenotype. In the family in our study, two members died from a fungal infection and a third presumably died from a similar cause. Further studies may clarify whether human CARD9 deficiency accounts only for recurrent mucosal infections or also accounts for an increased susceptibility to severe invasive fungal infections. In this consanguineous family, we cannot exclude the possibility that a second genetic defect may have contributed to a more severe phenotype in the deceased family members.
Unfortunately, we were unable to study viable cells from the family members in vitro because of logistical constraints. However, to understand the function of the human mutated CARD9 gene, we used an in vivo model with cells from Card9−/− mice and showed that the truncated human CARD9 protein fails to correct the dectin-1 signaling defect. In contrast, the human wild-type CARD9 protein restores the dectin-1–Card9 pathway in murine Card9−/− macrophages.
mice, stimulation of dendritic cells with the cell-wall component zymosan or whole C. albicans
cells results in a considerable reduction in the release of cytokines, including interleukin-2, interleukin-6, interleukin-10, and TNF-α
, and decreased numbers of Th17 cells, which are implicated in adaptive antifungal immunity.22,38
patients had significantly reduced numbers of Th17 cells, further supporting the notion that CARD9-mediated signaling contributes to Th17-cell differentiation. Th17 cells and their production of interleukin-17 have been shown to play a pivotal role in mucosal host defense against candidiasis in mice.56,57
Moreover, Eyerich et al. reported decreased numbers of Th17 cells in two sporadic cases of chronic mucocutaneous candidiasis, 58
but the role of these cells in human anti-fungal immunity remains elusive. If the lack of Th17 cells and their cytokines were critical for the pathogenesis of mucosal candidiasis, one could speculate that in patients with a low total CD4 count, such as in the low-CD4 syndrome, another rare primary immunodeficiency, or in patients with the acquired immunodeficiency syndrome, the lack of CD4 differentiation into Th17 cells is critical for maintaining the mucosal host defense against candida. Patients with the hyper-IgE syndrome, who lack Th17 cells because of heterozygous mutations in STAT3
, also have recurrent candidiasis.18,20
Whether Th17 cells are also implicated in the pathogenesis of candidiasis in APECED is currently being studied. The phenotype of susceptibility to fungal infections in human CARD9 deficiency serves as another example of a rare primary immunodeficiency that gives insight into the signaling pathways involved in immune regulation.