Role of IL-6 in Granulocytes
The shared gp130 component is the receptor signal transduction subunit for the IL-6 family of cytokines. In classical signaling, IL-6 binding to membrane-bound IL-6Rα (mIL-6Rα) induces gp130 homodimerization, whereas trans signaling involves IL-6 binding to a soluble IL-6Rα (sIL-6Rα) that interacts with gp130 expressed by the target cell. IL-6 is thought to influence granulocyte function, but because its effects on granulocytes are unclear, Wilkinson et al. (p. 3547) examined the ability of granulocytes to respond to classical and trans signaling. Circulating CD66b+ granulocytes from healthy donor blood lacked cell surface gp130 and failed to induce STAT1 and STAT3 phosphorylation, indicators of IL-6 responsiveness, following classical or trans signaling stimulation. Similarly, circulating murine neutrophils (Ly6G+) lacked gp130 expression and were refractory to IL-6 stimulation through either pathway. However, gp130 expression was observed in lineage−Sca-1+c-kit+ cells and both common myeloid progenitors (CMPs) and granulocyte-monocyte progenitors in murine bone marrow. Likewise, human hematopoietic stem progenitor cells, CMPs, and granulocyte-monocyte progenitors expressed gp130, and IL-6 stimulation resulted in significant phosphorylation of STAT3. Therefore, IL-6 may drive granulopoiesis by exerting its effects on granulocytic progenitors. These studies demonstrate that gp130 expression is lost during granulocyte maturation, suggesting that previously observed effects of IL-6 on mature granulocytes were indirect.
Tonic TCR Signaling and Immune Responses
T cell receptor signaling is critical during thymic development, but tonic signaling also impacts the activation state of the T cell in the periphery. In this issue, Milam et al. (p. 3429) generated an inducible knock-in mouse overexpressing the self-sensitizing voltage-gated sodium channel Scn5a, which is essential for signaling during thymocyte positive selection, to investigate how tuning TCR sensitivity to self-peptides alters mature T cell immune responses during infection. Peripheral CD4+ and CD8+ T cells from Scn5a (Scn5a+CD4-Cre+) knock-in mice displayed increased CD5 surface expression, which is known to fine-tune TCR signaling, and increased sensitivity of signaling at low levels of stimulation via TCR cross-linking. These data indicate that increased TCR sensitivity in peripheral T cells results in faster and more robust TCR-proximal signaling. To examine the functional consequences of Scn5a expression in the context of infection, Scn5a knock-in mice were crossed with transgenic mice whose CD4+ T cells recognize the Listeria protein listeriolysin O (LLO). Wild-type mice were coinjected with either LLO56 CD4+ T cells, which respond poorly in a primary Listeria infection, plus LLO118 CD4+ T cells, which respond robustly, or LLO56 T cells and LLO118.Scn5a+CD4-Cre+ T cells. Recipients of LLO118.Scn5a+CD4-Cre+ T cells demonstrated a decreased primary response to Listeria infection, as indicated by the recovery of nearly equal numbers of LLO118.Scn5a+Cre-CD4+ cells relative to LLO56 cells. Therefore, there is an inverse relationship between TCR sensitivity to self and the capacity for clonal expansion during a primary immune response. These studies demonstrate that tuning TCR sensitivity alters CD4+ T cell responses during infection and provide a link between a T cell’s ability to sense self and its response to infection.
IL-2 Mutein Can Selectively Inhibit Regulatory T Cells
Given the crucial role of IL-2 in the generation and homeostasis of regulatory T (Treg) cells, blockade of IL-2 signaling has been postulated to promote antitumor immune responses by inhibiting Treg cells. In this issue, Carmenate et al. (p. 3475) designed and characterized a novel IL-2 mutein that contained a mutation at the interface between IL-2 and the γc-chain, which enabled the mutein to bind to the αβ-chains of IL-2R but not to the γc-chain, thus attenuating IL-2R signaling. In vitro, the mutein inhibited Treg cell differentiation and expansion, antagonizing the effects of endogenous IL-2 that were produced upon anti-CD3 stimulation of naive CD4+ T cells. In an in vitro Treg cell suppression assay performed in the presence of the mutein, increasing numbers of Treg cells within a limited range restored effector T cell proliferation that was inhibited by the mutein in the absence of Treg cells. These data suggest that the mutein binds preferentially to the Treg cells. In vivo, administration of the mutein prior to melanoma and mammary tumor inoculation reduced tumor growth. Furthermore, the mutein reduced Treg cell infiltration in the tumors and tumor-draining lymph nodes. Finally, in silico mathematical simulations showed that an ideal mutein would reduce Treg cells and expand effector CD4+ and CD8+ memory T cells as well as NK cells. Overall, these results show that IL-2 muteins could be useful in cancer immunotherapy due to their ability to antagonize IL-2 binding to Treg cells while preserving IL-2-driven effector T cell function.
Early Intervention in HIV Preserves Mucosal B Cells
Persistent HIV-1 infection is associated with abnormalities in B cell phenotype and function within the blood and lymphoid tissues. Far less is known about whether these alterations also occur in the gut mucosa. Previous work demonstrated that initiation of combination antiretroviral therapy (cART) during the early phase of HIV-1 infection (e-ART) preserved intestinal lymphoid follicles when compared with cART initiated during the chronic phase (l-ART). In this issue, Planchais et al. (p. 3519) sought to determine if timing of cART could impact development of intestinal HIV-1–specific humoral responses. Examination of cell populations from freshly isolated rectal biopsies of patients with HIV-1 demonstrated that, compared with l-ART, e-ART was associated with an increased frequency of resting memory (RM) B cell populations and an expansion of CD4+ follicular helper T (TFH) cells. B cell follicle structure was preserved in e-ART patients, whereas B cell follicles from l-ART patients displayed some degree of disorganization, diffuse B cell distribution, and high levels of PD-L1expression within the B cell area. The frequency of mucosal HIV-1 gp140-reactive B cells was significantly higher in the e-ART group than in the l-ART group, and the phenotype of B cells differed between the two groups; mucosal B cells from e-ART patients were predominantly RM cells, whereas l-ART mucosal B cells were a mixture of RM and mature naive B cells. Importantly, the frequency of mucosal HIV gp140-reactive RM B cells was significantly higher in the e-ART group and positively correlated with the frequency of TFH cells. Moreover, mucosal B cells from the e-ART group released more HIV gp140-reactive IgG upon stimulation when compared with l-ART B cells. HIV-1 Env and Gag peptide–stimulated PBMCs from e-ART patients also released significantly more IL-21 than did l-ART PBMCs, which is critical to the generation of Ag-specific Abs and expansion of class-switched B and plasma cells. Thus, e-ART preserves gut lymphoid structures, which likely function as germinal centers; therefore, the timing of antiretroviral therapy critically influences the development of the anti-HIV Ab response.