We aimed to evaluate the mRNA and serum levels of OX40 in the peripheral blood of MS patients, and compare them with that of NMO patients and healthy controls. The results indicated increased expressions and serum levels of OX40 in the peripheral blood of patients with MS compared to NMO patients and healthy controls. Correlation analysis indicated a significant association between the mRNA expression and serum levels of OX40 in MS patients, but not in NMO subjects. Immune system over-activation has been proposed in the pathogenesis of MS, and several immunotherapeutic approaches have been used to treat the disease by blocking the co-stimulatory molecules involved in the activation of various immune cells during MS [
16,
34]. OX40 is expressed on activated T cells and Treg cells, while OX40L is expressed on the APCs, activated T cells, and endothelial and mast cells. The expression of both OX40 and OX40L is upregulated upon antigen presentation, CD28 and CD40L ligation to the corresponding receptors, and IFN-γ signaling. OX40 and OX40L interactions result in T cell proliferation and survival, promoted effector and memory T cell phenotypes, diminished regulatory signaling, enhanced cytokine production, and increased cell mobility [
16]. To date, most of the studies related to OX40-OX40L interaction in MS have been conducted on animal models. OX40L-deficient mice showed a milder EAE course, reduced T cell proliferation, and down-modulation of IL-2, IL-6, and IFN-γ production [
17]. OX40L expression was observed on the macrophages-microglia present in the CNS of EAE mice [
35]. Moreover, in vitro blockade of OX40L on macrophages-microglia resulted in an ex vivo suppression of T cell proliferation, suggesting that OX40-OX40L signaling can play a role in the reactivation of CNS T cells [
35]. OX40L expression was further identified on the EAE mice endothelial cells, implying the role of OX40-OX40L signaling in the migration and recruitment of immune cells to CNS [
36]. Using a soluble OX40R-Ig molecule to block OX40, resulted in a milder EAE course [
35]. In addition, anti-OX40L antibody ameliorated EAE [
36], but the treatment led to draining lymph nodes cells showing increased antigen-specific T cell proliferation as well as IFN-γ production. Nonetheless, amelioration of EAE course was correlated with a reduced number of T cells and infiltration of monocytes to the CNS of mice [
36]. These findings have implications for the treatment of EAE and possibly MS via blocking OX40-OX40L co-stimulatory signaling. Our analysis further revealed a significant increase in the EDSS value of MS subject (an indicator of disease severity) as the level of OX40 increased, further proposing a plausible therapeutic approach through the inhibition of OX40 or its signaling. On the other hand, natalizumab (a monoclonal antibody against very late antigen (VLA)-4)-treated MS patients had significantly decreased expressions of OX40 on CD4
+CD26
high T cells [
15]. We also found an increased expression and serum levels of OX40 in MS patients in comparison to healthy controls. Furthermore, OX40 was implicated in the NMO patients [
16]. NMO is defined as an inflammatory demyelinating disease of CNS, in which autoantibodies are developed against aquaporin 4 (AQP4) [
37]. It has been reported that active human NMO lesions contain OX40-expressing CD4
+ T cells. Moreover, expression of Ox40 has reflected the T cell activation and expansion, which was more profound compared to the expression of MS lesions of comparable activity [
38]. Our results also demonstrated increased expressions and serum levels of OX40 in NMO patients in comparison to the controls, although the difference was not significant. Nonetheless, NMO patients had significantly lower expressions and serum levels of OX40 compared with MS subjects. Therefore, it seems that OX40–OX40L signaling is more involved in the pathogenesis of MS, and humoral immunity (autoantibodies against AQP4) is predominantly involved in NMO pathogenesis. It has been reported that OX40 is up-regulated at the sites of autoimmunity and correlates with disease severity, especially in neurodegenerative diseases. Fu et al. showed that high levels of peripheral OX40+CD4+ T cells could be detected in systemic disease, but not in local disease, and there was no significant OX40 expression in peripheral blood [
39]. Fu et al. also have mentioned that blockade of the OX40–OX40L pathway in vivo generally improves the long‐term autoimmunity, mainly by preventing migration, moderating T cell polarization, altering inflammatory cytokine production, and preventing proliferation of active CD4
+ T cells [
39]. Active CD4+ T cells may play a critical role in MS pathogenesis [
40]. Fu et al. has been demonstrated that OX40–OX40L interaction can promote Th1 and Th2 mediated response, Th9 differentiation through the non-canonical NF-kB pathway [
39,
41]. Th1 and Th2 cytokine responses especially IFN-γ and IL-2 decreased in OX40L-deficient mice and treatment of wild type mice with anti-OX40L mAb (MGP-34) led to improve clinical symptom of EAE models [
17]. Studies in both preclinical and clinical trials have shown that blockade of OX40 or OX40L interaction can improve autoimmunity disease, especially EAE models, through inhibition of auto reactive T cells [
16,
39]. In previous study, Nohara et al. using variation dose of neutralizing ox40L mAB (RM134) have shown that significantly reduced the reservoir of OX40-expressing CD4 T cells led to inhibit migration of pathogenic T cells into central nervous system while it had little impact on the proliferation of Th1 cells in the draining lymph nodes (DLN) [
36].