Some years ago it has been described

Some years ago, it has been described that CD45RA+ CD62L+ naïve CD4+ T chlortetracycline receptor can be further differentiated into two distinct subpopulations by analysing their surface expression of CD31(Kimmig et al., 2002a; Kohler and Thiel, 2009; Kohler et al., 2005b). CD31+ thymicnaive CD4+ T cells are enriched in T cell receptor circles (TRECs), contain recent thymic emigrants (RTEs) display a polyclonal repertoire and might therefore be especially relevant in the generation of primary CD4+ T cell responses. In contrast, CD31- centralnaïve CD4+ T cells display a rather low TREC content, are oligoclonal, seem to be generated by homeostatic proliferation of naïve CD4+ T cells in the periphery and might contain autoreactive clones (Kohler and Thiel, 2009). In healthy donors the frequency and absolute count of CD31+ thymicnaive CD4+ T cells declines in correlation with age and the associated decrease of thymic function and these cells can be regarded as indicators of thymic function. In contrast, the absolute number of CD31- centralnaïve CD4+ T cells remains stable over time implying a peripheral regulation independent of thymic activity (Kohler et al., 2005b; Silva et al., 2017). Using these markers, we demonstrate that naïve CD4+ T cell homeostasis in an unselected cohort of MG patients is comparable to healthy controls. However, after exclusion of patients with potential confounding clinical characteristics, we detected an age-dependent increase in CD31- central naïve CD4+ T cells in MG that is not present in healthy donors. We demonstrate here that an aberrant production of naïve CD4+ T cells in thymoma can be detected in the peripheral blood by rather simple flow cytometric detection of CD31+ thymicnaive CD4+ T cells.
Materials and methods
Results
Discussion This study aims to investigate the naïve CD4+ T cell homeostasis in MG patients by determining the relative frequency among naïve CD4+ T cells and the absolute numbers of CD31+ thymicnaive and CD31- central naïve CD4+ T cells. First, we demonstrate that naïve CD4+ T cell homeostasis in an unselected cohort of MG patients is comparable to healthy controls. However, after exclusion of patients with potential confounders such as immunosuppressive treatment, thymectomy or thymoma, we detected an age-dependent increase in CD31- centralnaïve CD4+ T cells in MG patients not present in normal healthy controls. Furthermore, we demonstrate for the first time that an aberrant production of CD4+ T cells in thymoma patients can be revealed by flow cytometric assessment of CD31+ thymicnaive and CD31- centralnaïve CD4+ T cells in peripheral blood. It has been reported previously that absolute numbers of naïve and memory CD4+ T cells in the peripheral blood are similar between thymectomized and non-thymectomized MG patients, as well as healthy controls. In contrast, CD3+ T cell TREC levels were reduced in MG patients compared to healthy donors, an effect which was independent of previous thymectomy (Sempowski et al., 2001). Our study partly confirms these findings with respect to counts of naïve CD62L+ CD4+ T cells, where we also did not detect significant differences. In contrast to the above mentioned study (Sempowski, Thomasch, 2001), in our representative MG group the distribution of CD31+ thymicnaive CD4+ T cells is similar compared to healthy controls, indicating similar thymic output of naïve CD4+ T cells. Also CD31- centralnaïve CD4+ T cells were similar. On the other hand, when patients with potentially confounding clinical characteristics like thymoma or immunosuppressive therapy were excluded, we detected an increased count of peripherally expanded TREC low CD31− centralnaïve T cells in MG patients compared to healthy controls. While this finding might be in line with the previous observations (Sempowski et al., 2001) their results were based on the analysis of total CD3+ T cells and they do not provide a detailed description of their patient population and presence of potential confounders (Sempowski et al., 2001). Our results demonstrate that naïve CD4+ T cell homeostasis in MG patients is altered, although currently we cannot provide a clear mechanistic explanation. However, as in particular the subset of CD31- centralnaïve CD4+ T cells known to be clonally constricted most likely in response to self antigen in the periphery (Kohler and Thiel, 2009; Kohler et al., 2005b) is expanded, this might imply a predisposition to autoimmunity in MG patients. Therefore MG does share some similarity with multiple sclerosis and rheumatoid arthritis, where a dysbalanced thymic output and altered CD4+ T cell homeostasis is equally implicated in disease pathogenesis(Koetz et al., 2000) (Duszczyszyn et al., 2010; Haegert, 2011; Haegert et al., 2011). We can only speculate why this is in our cohort not reflected in lower percentages of CD31+ thymicnaive CD4+ T cells in MG patients. One reason might be an insufficient number of patients included in this analysis that results in lower sensitivity. Furthermore, we tested whether immunosuppressive drugs or clinical subtype (EOMG, LOMG or TMG) might influence the naive CD4+ T cell parameters in our setting. However, we could not detect any statistically relevant difference.