The dysregulation of Th1/Th2 cells has predominated investigations of the mechanism of allergic disease over the past 20 years. We sought to add an additional layer of complexity to the regulation of allergic inflammation. Recently, an increasing number of studies have already reported that the frequencies of circulating Tfh cells are increased in autoimmune diseases, which is accompanied by high levels of IL-21 and autoantibodies. However, the role of Tfh cells in IgE-mediated diseases is not well understood.
In the present study, we found no significant differences in the percentages of circulating CD4+CXCR5+ Tfh-like cells in peripheral blood between AR patients and healthy controls. This might be due to the predominately localized immune response in AR patients. Recently, Zhang Y N et al. confirmed that CD4+CXCR5+ Tfh cells accumulated and expanded in nasal polyp tissue and could promote IgG, IgA, and IgE production by naive B cells, and this could be blocked by depletion of IL-21 to some extent [24]. Blood CD4+CXCR5+ Tfh-like cells were shown to be circulating and to not fully reflect the local immunity. Another explanation was that blood CD4+CXCR5+ T cells were Tfh-like cells, and they were suggested to be resting cells with a memory phenotype [25]. Only a small proportion of blood CD4+CXCR5+ T cells expressed the activation molecules (ICOS and CD69) that are expressed by conventional follicular Tfh cells. Another recent study demonstrated that circulating CD4+CXCR5+ T cells can be defined as three subsets according to their differential expression of CXCR3 and CCR6 [10]. Only Th2-like CXCR5+ T cells (CXCR3−CCR6−) could induce naive B cells to produce IgE. Therefore, additional molecular markers could be used to define CD4+CXCR5+ T cells more precisely. The percentages of activated CD4+CXCR5+ T cells or Th2-like CXCR5+ T cells might make a difference in the severity of AR.
Here, we documented that CD4+CXCR5+ T cells in AR patients displayed decreased expression of the surface molecules SLAM and SAP. The decreased degree of SLAM and SAP expression reflected the disease severity to some extent by being negatively correlated with serum IgE. Since SLAM was expressed at higher levels on Th1 cells than on Th2 cells and could induce IFN-γ production [26], it was used as a marker of the Th2 to Th1 shift. Previous studies showed increased SLAM expression in Th1-predominant autoimmune diseases, such as multiple sclerosis [27] and rheumatoid arthritis [28]. Additionally, diminished expression of allergen-induced SLAM mRNA in PBMCs was observed in allergic rhinitis, and SLAM expression was recovered after specific immunotherapy (SIT) [13]. These reports supported our observation of the decreased expression of SLAM on CD4+CXCR5+ Tfh-like cells in AR.
We also discovered the decreased expression of SLAM on B cells and an equal decrease in the expression of both SLAM and SAP on CD4+CXCR5+ Tfh-like cells. SLAM primarily allows Tfh cells to adhere to B cells, and Tfh cell differentiation is induced by signals conveyed by B cells that sustain prolonged T-B conjugation times via SAP-dependent SLAM adhesion. Therefore, our observations suggested that there was a decrease in the formation of Tfh-B conjugates of SLAM-SLAM-SAP, indicating a change in the signal passed between Tfh cells and B cells.
As the most functional cytokine in Tfh cells, IL-21 showed levels that were significantly increased in AR patients and positively correlated with IgE. The role of IL-21 in IgE-mediated allergic diseases remains enigmatic, as conflicting results have been reported thus far. Salzer et al. reported that homozygous loss-of-function mutations in the IL-21 gene were associated with reduced numbers of CD19+ B cells and decreased serum IgG levels but an increased IgE concentration [29]. However, some in vitro experiments have shown that IL-21 enhances the production of IgE and promotes the proliferation of CD19+ B cells when they are stimulated with anti-CD40 and IL-4 or IL-13[30]. These discrepancies might be due to the pleiotropic effects of IL-21 and the complexity of the immune environment in vivo. Here, we speculate that high levels of serum IL-21 lead to high levels of IgE, which predominates type 2 immunity, in AR, and that the Th2 cytokines IL-4 and IL-13 also showed high concentrations. In addition to high levels of IL-21 in serum, a previous study indicated that there were higher frequencies of IL-21+ CD4+CXCR5+ T cells in allergic asthma patients [31]. Here, we speculated that CD4+CXCR5+ Tfh-like cells in AR patients could be more capable of producing IL-21, although the percentages of CD4+CXCR5+ Tfh-like cells showed no changes.
Because the decreased expression of SLAM and SAP was negatively correlated with the enhanced expression of serum IL-21, we examined the influence of SLAM on IL-21 production by CD4+CXCR5+ T cells. We observed that the engagement of SLAM resulted in a significant decrease in IL-21 expression in CD4+CXCR5+ T cells. This reduction in expression occurred at 48 h after stimulation and was maintained for 72 h, which revealed the prolonged effects. Downregulation of IL-21 occurred in a dose-dependent manner; the stronger the stimulation by SLAM was, the greater the reduction in the expression of IL-21 was. Conversely, insufficient/weak stimulation of CD4+CXCR5+ Tfh-like cells by SLAM resulted in more IL-21 production. This finding was consistent with our observation of the decreased expression of SLAM and high levels of IL-21 in AR patients. Therefore, we speculated that the low SLAM expression on CD4+CXCR5+ Tfh-like cells might contribute to the high levels of serum IL-21 in AR patients. Additionally, we reported increased percentages of B cells. This was in line with the high levels of IL-21 and IgE in serum in AR patients. IL-21 could drive more B cells to differentiate into plasma cells to secrete immunoglobulin, including IgE, which likely contributes to the pathogenesis of AR.
Based on a literature review, this report takes the initiative to focus on the role of SLAM on CD4+CXCR5+ Tfh-like cells. Our study provided evidence that decreased SLAM expression on CD4+CXCR5+ Tfh-like cells might upregulate the production of IL-21 to contribute to the pathogenesis of allergic rhinitis. Future studies need to be performed 1) to investigate SLAM expression on CD4+CXCR5+ Tfh-like cells in different allergic diseases, 2) to investigate SLAM expression on CD4+CXCR5+ Tfh-like cells in local tissues, and 3) to analyze the involvement of downstream factors that account for SLAM suppression of IL-21 expression. In terms of the value of this research, the data in the study extend the understanding of the pathogenesis of AR and identify a potential biomarker and therapeutic target.