The influence of IL-38 in the process of B cell differentiation

IL-38 is the latest discovered cytokine of the IL-1 family and has been added to the IL-36 subfamily. Since its discovery in 2001, increasing evidence suggests predominantly anti-inflammatory properties of IL-38, which are most likely exerted through three potential receptors, the IL-1 Receptor 1 (IL-1R1), IL-36 Receptor (IL-36R) and the IL-1 Receptor Accessory Protein Like 1 (IL-1RAPL1). However, to this date detailed knowledge of IL-38 functioning remains to be examined. Importantly, how IL-38 is processed, secreted from cells and the exact mechanisms of target receptor binding and intracellular signaling are not fully understood. Further, IL-38 has been associated with regulatory functions in autoimmune diseases like systemic lupus erythematosus (SLE) and psoriasis. At the same time however, connections between B cells as indispensable part of immunity and IL-38 remain rare. In this study we examined the influence of IL-38 in peripheral human blood B cells differentiating into antibody secreting cells using a three-step in vitro differentiation process. We first show that all potential IL-38 binding receptors are present on peripheral blood B cells on a gene expression level and remain detectable throughout B cell differentiation. Next, while B cells treated with exogenous IL-38 depict no differences in early B cell activation markers, the process of B cell differentiation revealed significant alterations in B cell phenotype created by IL-38 treatment. Predominantly on day 7 of the differentiation process, IL-38 treated B cells showed significantly reduced CD38 expression which depicts an important step in development towards plasma cells. We hypothesize that IL-38 acts antagonistically on the IL-1R1 pathway reducing Nuclear factor kappa B (NFκB) expression and consequently decreasing CD38 expression. Further IL-38 reduced early antibody production while increasing IgM secretion at the end stages of differentiation. Next, we repeated the differentiation assays under the influence of additional IL-21 stimulation to further enhance plasma cell development. In these experiments, the impact of IL-38 on B cell differentiation and immunoglobulin production were reduced, indicating a comparatively moderate relevance of IL-38 for B cell differentiation. We then examined how proliferation and cell death were impacted by exogenous IL-38 during B cell differentiation. IL-38 treatment alone significantly reduced B cell survival which was further augmented by IL-21 stimulation. We conclude that IL-38 and IL-21 act synergistically in promoting B cell apoptosis, also depicting an anti-inflammatory property of IL-38. Finally, using a siRNA we successfully performed an IL-38 knockdown experiment of human blood B cells reducing IL-38 expression to 44% measured on day 4 of B cell differentiation. In these experiments we observed reversed tendencies of CD38 expression compared to exogenous IL-38 treatment. Here, IL-38 knockdown cells showed increased CD38 expression indicating endogenous regulatory properties of IL-38 in B cell differentiation. Our project, for the first time proves direct effects of IL-38 on human B cells. The results support previous research of IL-38 to act anti-inflammatory as it seems to modulate B cell differentiation, survival, and immunoglobulin production in a down-regulatory manner. These findings pave way for more detailed research on the connection between B cell homoeostasis and IL-38 function.