Radhika Goenka

Cutting Edge: Dendritic Cell-Restricted Antigen Presentation Initiates the Follicular Helper T Cell Program but Cannot Complete Ultimate Effector Differentiation

Follicular helper T (TFH) cells are critical for germinal center (GC) formation. The processes that drive their generation and effector potential remain unclear. In this study, we define requirements for MHC class II APCs in murine TFH cell formation by either transiently ablating or restricting Ag presentation to dendritic cells (DCs). We find that cognate interactions with DCs are necessary and sufficient to prime CD4+ T cells toward a CXCR5+ICOS+Bcl6+ TFH cell intermediate. However, in the absence of additional APCs, these TFH cells fail to produce IL-21. Furthermore, in vitro priming of naive T cells by B cells engenders optimal production of IL-21, which induces a GC B cell transcriptional profile. These results support a multistep model for effector TFH cell priming and GC initiation, in which DCs are necessary and sufficient to induce a TFH cell intermediate that requires additional interactions with distinct APCs for full effector function.

BLyS inhibition eliminates primary B cells but leaves natural and acquired humoral immunity intact

We have used an inhibiting antibody to determine whether preimmune versus antigen-experienced B cells differ in their requisites for BLyS, a cytokine that controls differentiation and survival. Whereas in vivo BLyS inhibition profoundly reduced naïve B cell numbers and primary immune responses, it had a markedly smaller effect on memory B cells and long-lived plasma cells, as well as secondary immune responses. There was heterogeneity within the memory pools, because IgM-bearing memory cells were sensitive to BLyS depletion whereas IgG-bearing memory cells were not, although both were more resistant than naïve cells. There was also heterogeneity within B1 pools, as splenic but not peritoneal B1 cells were diminished by anti-BLyS treatment, yet the number of natural antibody-secreting cells remained constant. Together, these findings show that memory B cells and natural antibody-secreting cells are BLyS-independent and suggest that these pools can be separately manipulated.

A role for IL-27p28 as an antagonist of gp130-mediated signaling

The heterodimeric cytokine interleukin 27 (IL-27) signals through the IL-27Rα subunit of its receptor, combined with gp130, a common receptor chain used by several cytokines, including IL-6. Notably, the IL-27 subunits p28 (IL-27p28) and EBI3 are not always expressed together, which suggests that they may have unique functions. Here we show that IL-27p28, independently of EBI3, antagonized cytokine signaling through gp130 and IL-6-mediated production of IL-17 and IL-10. Similarly, the ability to generate antibody responses was dependent on the activity of gp130-signaling cytokines. Mice transgenic for expression of IL-27p28 showed a substantial defect in the formation of germinal centers and antibody production. Thus, IL-27p28, as a natural antagonist of gp130-mediated signaling, may be useful as a therapeutic for managing inflammation mediated by cytokines that signal through gp130.

Asymmetric B Cell Division in the Germinal Center Reaction

Stochastic or Asymmetric Fate Determination? During an adaptive immune response, B lymphocytes rapidly divide and differentiate into effector cell populations, including antibody-secreting plasmablasts and memory B cells. Many also change the class of antibody they secrete, through a process called isotype switching. During this process, some cells die. Whether cells acquire these different fates in a stochastic or programmed manner, however, is unclear. Duffy et al. (p. 338, published online 5 January) used single-cell tracking to determine the times to division, differentiation into a plasmablast, isotype switching, and death of stimulated B lymphocytes. Statistical analysis and mathematical modeling revealed that these cell-fate decisions appear to be the result of random clocks: Which clock went off first (division, differentiation, or death), determined the fate of the cell. Barnett et al. (p. 342, published online 15 December) sought to determine whether asymmetrical cell division, which is thought to contribute to effector cell-fate decisions in T cells, may be at work in B lymphocytes. Indeed, factors important for the initiation and maintenance of germinal center B lymphocyte identity, along with an ancestral polarity protein, were asymmetrically distributed and maintained their asymmetry during cell division. Germinal center B cells undergo asymmetric cell division. Lifelong antibody responses to vaccination require reorganization of lymphoid tissue and dynamic intercellular communication called the germinal center reaction. B lymphocytes undergo cellular polarization during antigen stimulation, acquisition, and presentation, which are critical steps for initiating humoral immunity. Here, we show that germinal center B lymphocytes asymmetrically segregate the transcriptional regulator Bcl6, the receptor for interleukin-21, and the ancestral polarity protein atypical protein kinase C to one side of the plane of division, generating unequal inheritance of fate-altering molecules by daughter cells. Germinal center B lymphocytes from mice with a defect in leukocyte adhesion fail to divide asymmetrically. These results suggest that motile cells lacking constitutive attachment can receive provisional polarity cues from the microenvironment to generate daughter cell diversity and self-renewal.

Local BLyS production by T follicular cells mediates retention of high affinity B cells during affinity maturation

BLyS expression by GC follicular T cells is required for the efficient selection of high-affinity GC B cells.

Homeostatic control of B lymphocyte subsets

Lymphocyte homeostasis poses a multi-faceted biological puzzle, because steady pre-immune populations must be maintained at an acceptable steady state to yield effective protection, despite stringent selective events during their generation. In addition, activated, memory and both short- and long-term effectors must be governed by independent homeostatic mechanisms. Finally, advancing age is accompanied by substantial changes that impact the dynamics and behavior of these pools, leading to cumulative homeostatic perturbations and compensation. Our laboratory has focused on the overarching role of BLyS family ligands and receptors in these processes. These studies have led to a conceptual framework within which distinct homeostatic niches are specified by BLyS receptor signatures, which define the BLyS family ligands that can afford survival. The cues for establishing these receptor signatures, as well as the downstream survival mechanisms involved, are integrated with cell extrinsic inputs via cross talk among downstream mediators. A refined understanding of these relationships should yield insight into the selection and maintenance of B cell subsets, as well as an appreciation of how homeostatic mechanisms may contribute to immunosenescence.

New roles for the BLyS/BAFF family in antigen-experienced B cell niches

BLyS family members govern selection and survival of cells in the pre-immune B cell compartment, and emerging evidence suggests similar roles in antigen-experienced B cell pools. We review the features of this family, with particular emphasis on recent findings of how BLyS influences affinity maturation in germinal centers, which lie at the intersection of the pre-immune and antigen-experienced B cell compartments. We propose a model whereby tolerogenic selection at the transitional stage and affinity maturation in the germinal center employ the same BLyS driven mechanism.

Chronic Spinal Cord Injury Impairs Primary Antibody Responses but Spares Existing Humoral Immunity in Mice

Spinal cord injury (SCI) results in immune depression. To better understand how injury inhibits humoral immunity, the effects of chronic thoracic SCI on B cell development and immune responses to thymus-independent type 2 and thymus-dependent Ags were determined. Mice received complete crush injury or control laminectomy at either thoracic level 3, which disrupts descending autonomic control of the spleen, or at thoracic level 9, which conserves most splenic sympathetic activity. Although mature B cell numbers were only mildly reduced, bone marrow B cell production was transiently but profoundly depressed immediately after injury. Despite the return of normal B cell production 4 wk after SCI, mice receiving thoracic level 3 injury showed a significant reduction in their ability to mount primary thymus-independent type 2 or thymus-dependent immune responses. The latter were marked by decreases in germinal center B cells as well as class-switched high-affinity Ab-secreting cells. Importantly, injury did not affect affinity maturation per se, pre-existing B cell memory, or secondary humoral immune responses. Taken together, these findings show that chronic high thoracic SCI impairs the ability to mount optimal Ab responses to new antigenic challenges, but spares previously established humoral immunity.

The role of B lymphocyte stimulator in B cell biology: implications for the treatment of lupus.

B lymphocyte stimulator (BLyS; also known as B cell activating factor (BAFF)) plays a key role in peripheral B cell tolerance. Mounting evidence indicates that B cell tolerance can be either broken or modulated by deliberately manipulating BLyS levels, and belimumab, a BLyS-neutralizing antibody, was recently approved for the treatment of systemic lupus erythematosus (SLE). Thus, intense investigation has focused on understanding how therapeutics targeting BLyS may work, and accumulating evidence suggests multiple points of action. BLyS signaling, in conjunction with B cell receptor (BCR) signaling, determines the size and quality of the mature primary B cell compartment. Moreover, BLyS family members play roles in antigen-experienced B cell selection and differentiation. Together, these findings have implications for the continued development of novel therapeutics that target BLyS.

Interferon γ-induced intratumoral expression of CXCL9 alters the local distribution of T cells following immunotherapy with Listeria monocytogenes

The ability of Listeria monocytogenes-based anticancer vaccines to induce tumor regression depends on the responsiveness of malignant cells to interferon γ (IFNγ). Inhibition of IFNγ limits the recruitment of T cells to the tumors of vaccinated mice. We hypothesized that vaccination with immunotherapeutic L. monocytogenes induces the IFNγ-dependent production of chemokines that regulate the migration of tumor-infiltrating T cells. To gain further insights into this issue, we examined the chemokine responses of a transplantable, human papillomavirus (HPV)-immortalized murine tumor model (TC-1) following the administration of a L. monocytogenes-based immunotherapeutic agent that expresses E7 from HPV-16. Here, we report that the administration of L. monocytogenes-based anticancer vaccines increases the secretion of chemokine (C-X-C motif) ligand 9 (CXCL9), and CXCL10 by tumors, hence favoring the recruitment of T cells bearing the cognate chemokine (C-X-C motif) receptor 3 (CXCR3). Furthermore, the expression of CXCL9, but not CXCL10, in TC-1 tumors was significantly reduced upon anti-IFNγ antibody treatment. CXCL9 was highly expressed by TC-1 cells following the administration of IFNγ and tumor necrosis factor α (TNFα), in vitro. Moreover, the inhibition of CXCL9 in TC-1 cells reduced the proportion of CD8+ T cells infiltrating tumors in vaccinated mice, while increasing that of CD4+ T cells, thus altering T-cell subset distribution. We conclude that the administration of L. monocytogenes-based anticancer vaccines regulates TH1 chemokine responses and that malignant cells are an important source of these chemokines.