Despite myriad of distinct biological pathways have been implicated in its pathophysiology, it is generally believed that IBD is a result of a maladaptive immune response to gut-resident commensal bacteria in a genetically susceptible host
[1]. Inflammation in CD seems to be mainly driven by T
h1 responses, whereas T
h2 responses dominate the pathobiology of UC
[2]. Nevertheless, additional lymphocytes, such as innate lymphoid cells and T
h17 cells have also arisen as key players in the pathogenesis of IBD
[3]. Specifically, abnormal pro-inflammatory CD4
+ T-cell responses mediated by effector T
h1, T
h2, or T
h17 cells disrupt homeostasis and causes IBD by outweighing anti-inflammatory CD4
+ T-cell responses orchestrated by T regulatory (T
reg) cells
[4]. Even though T-cell system is predominant in studies concerning the IBD pathogenesis and therapeutic approach, emerging data suggest a role of B-cell lineage in IBD as well. Firstly, humoral homeostasis seems to be impaired in IBD. For instance, it has been shown that production of functional, dimeric immunoglobulin A (IgA) is impaired in patients with IBD
[5]. As IgA exerts local anti-inflammatory effects by coating commensal bacteria after undergoing transepithelial translocation in gut, its depletion favors gut inflammation
[6]. Moreover, B-cell expression of the pro-inflammatory cytokine IL-8, as well as production of mucosal IgG in gut are upregulated in IBD, thereby further promoting inflammation
[7][8]. Accordingly, expression of both IL-8 and TLR-2 in IBD patients positively correlated with CD activity
[8][9][10].
In murine IBD model, poorly regulated B-cells have been shown to exacerbate inflammation by blocking T
reg cell function
[11][12]. Moreover, B-cells promote ileitis in UC by producing epithelial cell-specific autoantibodies
[11][13]. Mucosal IgG in IBD can be directed against microbial elements, such as anti-saccharomyces-cerevisiae antibodies (ASCA) and anti-flagellin antibodies, or autoantigens, such as anti-neutrophil cytoplasmic antibodies (ANCA) and anti-epithelial antibodies
[14][15]. Notably, the role of anti-granulocyte macrophage colony-stimulating factor (anti-GM-CSF) in IBD is fairly complex, yet the presence of anti-GM-CSF in the setting of IBD is associated with ileal phenotype and intricated behavior of the disease
[16]. The latter observation is in line with preclinical data, as NOD2 KO mice treated with anti-GM-CSF antibodies develop transmural ileitis subsequent to NSAID exposure
[16]. Alterations of the B-cell lineage in IBD are less obvious, and more complex for that matter, than those associated with derangement of T-cell system
[17]. Acknowledgement of poor understanding of the complexity of B-cell responses in IBD allows us to evaluate recently failed therapeutic attempts in UC involving the CD20-targeting agent rituximab more critically
[18]. Namely, negative outcomes of therapeutic approaches including rituximab should not discourage from considering B-cells as potential therapeutic target in IBD, especially since unresponsiveness to rituximab can be also observed in certain cases of B-cell-related autoimmune disorders such as rheumatoid arthritis (RA) and vasculitis
[19][20]. For example, in certain group of patients, paradoxical pro-inflammatory manifestations can occur subsequent to rituximab administration
[21][22]. While dysfunctional B-cell lineage can promote autoimmunity via autoreactive, long-lived plasma cells, regulatory B-cells can attenuate inflammation too. For instance, anti-CD20-treated mice deficient in peripheral B-cells failed to undergo spontaneous recovery and even developed chronic disease in a model of murine autoimmune encephalomyelitis
[23][24]. In addition, B-cells may produce anti-inflammatory IL-10 but may also promote the anti-inflammatory effect of T
reg cells
[25]. Even more perplexing is the fact that tissue resident plasma cells do not express CD20 and thus cannot be targeted by rituximab
[26]. In the setting of IBD, in vitro experiment showed that plasma cells subset expanded in the mucosa of IBD patients and was resistant to rituximab-induced apoptosis
[27]. Nevertheless, evidence of rituximab in IBD is conflicting, as some studies showed that rituximab could improve colonic inflammation, whereas case reports showed that rituximab could trigger colitis
[28][29][30][31]. In fact, a retrospective cohort study showed that patients on rituximab have a sixfold increased risk of developing IBD compared to the general population
[32]. Finally, a phase II randomized controlled trial, in which effects of rituximab on UC patients was assessed, showed no significant effect on inducing remission in moderately active UC not responding to oral steroids with possible short-term response that was not sustained
[28].