Background
Multiple sclerosis (MS) is a chronic demyelinating disease of central nervous system (CNS) characterized by destruction of myelin sheath and nerve axons [1]. CD4+ T cells are considered a key player in MS pathogenesis. Different subsets of CD4+ T cells orchestrate an inflammatory wave to initiate and progress MS [2, 3]. Additionally, B cells are known to play an important role in MS through antibody production or influencing autoreactive T cell generation [4-6]. T follicular helper (Tfh) cells, a newly described CD4+ T cell subset that resides in the germinal center (GC), are specialized for B cell help and antibody mediated immune responses [7]. Tfh cells possess unique phenotypic attributes and express chemokine receptor type 5 (CXCR5), inducible T cell co-stimulator (ICOS), and program death-1 (PD-1) at their surfaces, all controlled by a B cell lymphoma 6 transcription factor (Bcl-6) [7-11]. IL-21 is the signature cytokine produced by Tfh cells, which is required for Tfh maintenance, B cell differentiation and antibody production [11-13]. Conversely, follicular regulatory T (Tfreg) cells have been recently identified as a novel subtype of regulatory T cells that share some common phenotypes with Tfh, along with conventional Treg markers, such as forkhead box P3 (FoxP3) and CD25 [14, 15]. Similar to the canonical Treg, Tfreg cells also secrete IL-10 [16] and their basic role is to regulate the functional activity of Tfh cells in the GC [17, 18].
Since their discovery, Tfh and Tfreg cells have become crucial parts of the antigen specific immune responses and self-tolerance, which led to explore their relevance in human health and disease [17, 19]. The characterization of bona fide Tfh and Tfreg in humans has been poorly studied because of sampling difficulties. Therefore, studies have focused on their counterparts in peripheral blood, termed as circulating Tfh (cTfh) cells [20]. These cTfh cells are identified in the memory T cell chamber, sharing common phenotypes with GC-Tfh except Bcl-6 [21]. Like resident Tfh, cTfh cells are also capable of providing help to B cells [22]. Recently, cTfh cells have been reported to exert more potent cytokine mediated immune responses than GC-Tfh [23]. Moreover, armed with memory-like properties, cTfh cells may provide more rapid cellular immune response and undergo homeostatic proliferation, where they show phenotypic heterogeneity, accessing a broader range of immune compartments and expressing a variety of surface markers and secreting an array of cytokines [24]. Morita et al. initially described the existence of Th1 (CXCR3) and Th17 (CCR6) related surface markers on cTfh cells. cTfh cells were divided into cTfh1 (CXCR3+CCR6), cTfh2 (CXCR3CCR6), and cTfh17 (CXCR3CCR6+), with each involved in producing different sets of cytokines, namely IFN-γ, IL-4, and IL-17 respectively, as well as secreting IL-21 [25]. These diverse phenotypic markers and cytokines collectively reshape cTfh cells to provide more robust help to B cells and induce high affinity maturation of plasma cells and antibody production [20, 25].
Emerging evidence suggests that Tfh cells should be tightly controlled to maintain immune tolerance and avoid autoimmune responses. Uncontrolled activation of Tfh cells and deactivation of Tfreg cells lead to a breakdown of self-tolerance. Imbalance between Tfh and Tfreg cells has been found in number of autoimmune diseases, including systemic lupus erythematous (SLE), rheumatoid arthritis (RA), and myasthenia gravis (MG) [26-29]. Moreover, subsequent polarization of Tfh subsets has reportedly been involved in several autoimmune diseases, such as SLE, RA and IgG4-related disease which are associated with autoantibody production [30-32]. The relevance of Tfh and Tfreg cells has recently been recognized in MS patients, suggesting their potential roles in MS pathogenesis [33, 34]. To address this issue we investigated the frequency of cTfh (CD4+CXCR5+PD-1+) cells and their subsets, as well as cTfreg (CD4+CXCR5+PD-1+FoxP3+CD25+) cells in MS patients compared to healthy controls (HC). We also investigated the frequency of respective cytokines secreted by these cellular subtypes. We found that patients with MS display alterations to their cTfh and cTfreg cells, both in terms of their frequencies and cytokine secretions.