CD4+ T Cell–Derived IL-21 and Deprivation of CD40 Signaling Favor the In Vivo Development of Granzyme B–Expressing Regulatory B Cells in HIV Patients

Abstract

IL-21 can induce both plasma cells and regulatory B cells. In this article, we demonstrate that untreated HIV patients display CD4+ T cells with enhanced IL-21 expression and high in vivo frequencies of regulatory B cells overexpressing the serine protease granzyme B. Granzyme B–expressing regulatory B cells (GraB cells) cells from HIV patients exhibit increased expression of CD5, CD43, CD86, and CD147 but do not produce IL-10. The main functional characteristic of their regulatory activity is direct granzyme B–dependent degradation of the TCR-ζ–chain, resulting in significantly decreased proliferative T cell responses. Although Th cells from HIV patients secrete IL-21 in a Nef-dependent manner, they barely express CD40L. When culturing such IL-21+CD40L− Th cells with B cells, the former directly induce B cell differentiation into GraB cells. In contrast, the addition of soluble CD40L multimers to T cell/B cell cultures redirects B cell differentiation toward plasma cells, indicating that CD40L determines the direction of IL-21–dependent B cell differentiation. As proof of principle, we confirmed this mechanism in a patient lacking intact CD40 signaling due to a NEMO mutation. The majority of peripheral B cells from this patient were GraB cells and strongly suppressed T cell proliferation. In conclusion, GraB cells represent potent regulatory B cells in humans that are phenotypically and functionally distinct from B10 cells and occur in early HIV infection. GraB cells may contribute significantly to immune dysfunction in HIV patients, and may also explain ineffective Ab responses after vaccination. The use of soluble CD40L multimers may help to improve vaccination responses in HIV patients.

The Biological Effects of IL-21 Signaling on B-Cell-Mediated Responses in Organ Transplantation

“…This causes phosphorylation of tyrosine residues on IL-21R/γc, providing docking sites for STAT proteins and other signaling molecules (26). On recruitment, STATs are phosphorylated and form homodimers or heterodimers, which translocate into the nucleus and modulate expression of the target genes (27), which regulate B-cells, such as B-cell-induced maturation protein-1 (Blimp-1) (28), B-cell lymphoma (BCL)-6 (29), activation-induced cytidine deaminase (AID) (30), granzyme (31), somatic hypermutation (SHM) (32), paired box 5 (Pax5) (33), X-box-binding protein 1 (XBP-1) (34), and Bim (35). IL-21 mediates B-cell proliferation, immunoglobulin (Ig) production, and apoptotic functions mainly through the potent effects of STAT3 and/or STAT1 activation but also, to a lesser extent, through STAT4 and STAT5 (36–39) (Figure 1).…”

“…Interleukin-21 can induce BCR-stimulated human B-cells to differentiate into granzyme B-expressing cytotoxic cells (GrB) in a STAT3-dependent manner in the absence of a CD40 signal (31, 77, 86–88). GrB + B-cell numbers are dependent on IL-21 production, and increasing doses of anti-IL-21 decreased the number of GrB-expressing B-cells in co-culture systems (78).…”

Regulatory B Cells with a Partial Defect in CD40 Signaling and Overexpressing Granzyme B Transfer Allograft Tolerance in Rodents

“…Interestingly, it has been reported in humans that granzyme B + B cells accumulated specifically in tissues and have been suggested to play a critical role in early antiviral immune responses, in the regulation of autoimmune mechanisms, and in cancer immunosurveillance (45)(46)(47). Additionally, it has recently been shown that BCR and IL-21 signaling without an efficient CD40-CD40L ligation leads to the generation of granzyme B + B cells that induce immune dysfunction in solid tumor eradication (45,(48)(49)(50) or in HIV patients (51). Therefore, in our model, the regulatory B cells when they reencountered the alloantigens in the spleen and because of their defect in CD40 signaling may lead to the generation of granzyme B + plasmablast-like regulatory B cells that accumulated particularly in the graft tissue.…”

Biological effects of IL-21 on different immune cells and its role in autoimmune diseases

“…It was also shown that IL-21 signaling affects the interactions between T cells and B cells and repeal protective humoral immunity to Malaria (45). Interestingly, recent studies have demonstrated that despite the fact that IL-21 can provide apoptotic signals for the B cells, it can also induce BCR-stimulated human B cells to differentiate into Granzyme B-expressing cytotoxic cells against tumor cells in the absence of CD40 signal in a STAT3-dependent fashion (46)(47)(48)(49). Based on these conflicting studies, it is believed that IL-21 may have a role in early stages of infection by induction of B cell proliferation and amplification of the humoral immune response while inducing apoptosis of B cells as a mechanism for eliminating inappropriately activated autoreactive B cells (24).…”