Huanbin Xu

Cutting edge: induction of B7-H4 on APCs through IL-10: novel suppressive mode for regulatory T cells.

Multiple modes of suppressive mechanisms including IL-10 are thought to be implicated in CD4+CD25+ regulatory T (Treg) cell-mediated suppression. However, the cellular source, role, and molecular mechanism of IL-10 in Treg cell biology remain controversial. We now studied the interaction between Treg cells and APCs. We demonstrate that Treg cells, but not conventional T cells, trigger high levels of IL-10 production by APCs, stimulate APC B7-H4 expression, and render APCs immunosuppressive. Initial blockade of B7-H4 reduces the suppressive activity mediated by Treg cell-conditioned APCs. Further, APC-derived, rather than Treg cell-derived, IL-10 is responsible for APC B7-H4 induction. Therefore, Treg cells convey suppressive activity to APCs by stimulating B7-H4 expression through IL-10. Altogether, our data provide a novel cellular and molecular mechanism for Treg cell-mediated immunosuppression at the level of APCs, and suggest a plausible mechanism for the suppressive effect of IL-10 in Treg cell-mediated suppression.

Bone marrow and the control of immunity

Bone marrow is thought to be a primary hematopoietic organ. However, accumulated evidences demonstrate that active function and trafficking of immune cells, including regulatory T cells, conventional T cells, B cells, dendritic cells, natural killer T (NKT) cells, neutrophils, myeloid-derived suppressor cells and mesenchymal stem cells, are observed in the bone marrow. Furthermore, bone marrow is a predetermined metastatic location for multiple human tumors. In this review, we discuss the immune network in the bone marrow. We suggest that bone marrow is an immune regulatory organ capable of fine tuning immunity and may be a potential therapeutic target for immunotherapy and immune vaccination.

IL-17-producing innate lymphoid cells are restricted to mucosal tissues and are depleted in SIV-infected macaques

Innate lymphoid cells (ILCs) are an emerging subset of lymphocytes involved in surveillance against virally infected cells. Here, we show CD3−CD8high lymphocytes in macaque blood include major subsets of ILCs including natural killer (NK) cells expressing CD16, NKp46, and NKG2A, but also populations of ILCs in mucosal tissues having different properties. One ILC subset secreted interleukin (IL)-17 (ILC17), but these were restricted to mucosal tissues. Some mucosal ILC17 cells expressed classical NK-cell markers, but little NKG2A or NKG2D. Some ILC17 cells secreted IL-22 and tumor necrosis factor-α, but few produced interferon (IFN)-γ or contained granzyme B. IL-17 production by ILCs was induced by IL-6, transforming growth factor-β, and IL-23. Further, simian immunodeficiency virus (SIV) infection resulted in a significant loss of ILC17 cells, especially in the jejunum, which persisted throughout SIV infection. These findings indicate that ILC17 cells may be involved in innate mucosal immune responses, and their loss may contribute to loss of intestinal mucosal integrity and disease progression in human immunodeficiency virus (HIV)/SIV infection.

Monitoring α4β7 integrin expression on circulating CD4+ T cells as a surrogate marker for tracking intestinal CD4+ T-cell loss in SIV infection

Intestinal CD4+ T cells are rapidly and profoundly depleted in human immunodeficiency virus (HIV)-infected patients and simian immunodeficiency virus (SIV)-infected macaques. However, monitoring intestinal cells in humans is difficult, and identifying surrogate markers in the blood, which correlate with loss or restoration of intestinal CD4+ T cells could be helpful in monitoring the success of therapeutic strategies and vaccine candidates. Recent studies indicate HIV utilizes the intestinal homing molecule α4β7 for attachment and signaling of CD4+ T cells, suggesting this molecule may have a central role in HIV pathogenesis. Here, we compared β7HIGH integrin expression on CD4+ T cells in blood with loss of CD4+ T cells in the intestine of macaques throughout SIV infection. The loss of β7HIGH CD4+ T cells in blood closely paralleled the loss of intestinal CD4+ T cells, and proved to be a more reliable marker of intestinal CD4+ T-cell loss than monitoring CCR5+ memory CD4+ T cells. These data are consistent with a recent hypothesis that α4β7 has a role in the selective depletion of intestinal CD4+ T cells, and indicate that monitoring β7HIGH expression on CD4+ T cells in the blood may be a useful surrogate for estimating intestinal CD4+ T cell loss and restoration in HIV-infected patients.

Th17 Cells Are Preferentially Infected Very Early after Vaginal Transmission of SIV in Macaques

The difficulty in detecting rare infected cells immediately after mucosal HIV transmission has hindered our understanding of the initial cells targeted by the virus. Working with the macaque simian immunodeficiency virus (SIV) vaginal challenge model, we developed methodology to identify discrete foci of SIV (mac239) infection 48 hr after vaginal inoculation. We find infectious foci throughout the reproductive tract, from labia to ovary. Phenotyping infected cells reveals that SIV has a significant bias for infection of CCR6+ CD4+ T cells. SIV-infected cells expressed the transcriptional regulator RORγt, confirming that the initial target cells are specifically of the Th17 lineage. Furthermore, we detect host responses to infection, as evidenced by apoptosis, cell lysis, and phagocytosis of infected cells. Thus, our analysis identifies Th17-lineage CCR6+ CD4+ T cells as primary targets of SIV during vaginal transmission. This opens new opportunities for interventions to protect these cells and prevent HIV transmission.

Increased B7-H1 Expression on Dendritic Cells Correlates with Programmed Death 1 Expression on T Cells in Simian Immunodeficiency Virus-Infected Macaques and May Contribute to T Cell Dysfunction and Disease Progression

Suppression of dendritic cell (DC) function in HIV-1 infection is thought to contribute to inhibition of immune responses and disease progression, but the mechanism of this suppression remains undetermined. Using the rhesus macaque model, we show B7-H1 (programmed death [PD]-L1) is expressed on lymphoid and mucosal DCs (both myeloid DCs and plasmacytoid DCs), and its expression significantly increases after SIV infection. Meanwhile, its receptor, PD-1, is upregulated on T cells in both peripheral and mucosal tissues and maintained at high levels on SIV-specific CD8+ T cell clones in chronic infection. However, both B7-H1 and PD-1 expression in SIV controllers was similar to that of controls. Expression of B7-H1 on both peripheral myeloid DCs and plasmacytoid DCs positively correlated with levels of PD-1 on circulating CD4+ and CD8+ T cells, viremia, and declining peripheral CD4+ T cell levels in SIV-infected macaques. Importantly, blocking DC B7-H1 interaction with PD-1+ T cells could restore SIV-specific CD4+ and CD8+ T cell function as evidenced by increased cytokine secretion and proliferative capacity. Combined, the results indicate that interaction of B7-H1–PD-1 between APCs and T cells correlates with impairment of CD4+ Th cells and CTL responses in vivo, and all are associated with disease progression in SIV infection. Blockade of this pathway may have therapeutic implications for HIV-infected patients.

Mucosal Immunology of HIV Infection

Recent advances in the immunology, pathogenesis, and prevention of human immunodeficiency virus (HIV) infection continue to reveal clues to the mechanisms involved in the progressive immunodeficiency attributed to infection, but more importantly have shed light on the correlates of immunity to infection and disease progression. HIV selectively infects, eliminates, and/or dysregulates several key cells of the human immune system, thwarting multiple arms of the host immune response, and inflicting severe damage to mucosal barriers, resulting in tissue infiltration of ‘symbiotic’ intestinal bacteria and viruses that essentially become opportunistic infections promoting systemic immune activation. This leads to activation and recruitment or more target cells for perpetuating HIV infection, resulting in persistent, high‐level viral replication in lymphoid tissues, rapid evolution of resistant strains, and continued evasion of immune responses. However, vaccine studies and studies of spontaneous controllers are finally providing correlates of immunity from protection and disease progression, including virus‐specific CD4+ T‐cell responses, binding anti‐bodies, innate immune responses, and generation of antibodies with potent antibody‐dependent cell‐mediated cytotoxicity activity. Emerging correlates of immunity indicate that prevention of HIV infection may be possible through effective vaccine strategies that protect and stimulate key regulatory cells and immune responses in susceptible hosts. Furthermore, immune therapies specifically directed toward boosting specific aspects of the immune system may eventually lead to a cure for HIV‐infected patients.

Persistent Simian Immunodeficiency Virus Infection Drives Differentiation, Aberrant Accumulation, and Latent Infection of Germinal Center Follicular T Helper Cells

ABSTRACT CD4+ follicular T helper (Tfh) cells play a prominent role in humoral immune responses, but the mechanisms of their accumulation and infection in AIDS remain unclear. Here we found that germinal center (GC) Tfh cells, defined here as CXCR5+ PD-1HIGH CD4+ T cells, do not express the HIV coreceptor CCR5 yet serve as a latent reservoir in GCs. With disease progression, an expansion of GC Tfh cells is accompanied by increases in dysfunctional CD8+ T cells. In contrast, Tfh precursor (CXCR5− CD4+ T) cells in lymph nodes do express CCR5 and differentiate into GC Tfh cells following interleukin-6 (IL-6) and IL-21 stimulation, and viral DNA is detectable in fully differentiated GC Tfh cells ex vivo. This suggests that SIV-infected GC Tfh cells may be derived from Tfh precursor cell subsets that become infected in marginal zones and then migrate into GCs as fully mature GC Tfh cells that serve as persistent virus reservoirs. These findings suggest that viral persistence in lymph nodes drives compensatory differentiation, aberrant accumulation, and latent infection of GC Tfh cells, resulting in marked impairment of humoral immune responses. IMPORTANCE Generation of antibodies that can effectively eliminate viruses requires interactions of B cells with highly specialized T cells in GCs of lymphoid tissues called follicular T helper cells. Here we show that in simian immunodeficiency virus infection, these cells are initially infected in a precursor stage that leads to alterations in their homing, accumulation, and function that may be responsible for the inability of human immunodeficiency virus-infected patients to generate effective antibody responses.

PD-1HIGH Follicular CD4 T Helper Cell Subsets Residing in Lymph Node Germinal Centers Correlate with B Cell Maturation and IgG Production in Rhesus Macaques

CD4+ T follicular helper (TFH) cells guide development and maturation of B cells and are crucial for effective antibody responses. Here we found rhesus macaque TFH cells, defined as CXCR5+CD4 T cells, contain two major populations: PD-1INT and PD-1HIGH cells. Of these, PD-1HIGHCD4+ T cells highly co-express ICOS but little CCR7, and reside in lymph node germinal centers (GCs), but not in blood. These cells secrete IL-21 and express transcriptional factor Bcl-6 at higher levels than CXCR5+PD-1INTCD4+ T cells. In addition, the frequency of PD-1HIGHCD4+ T cells is low in lymph nodes of newborns, but increases with age. Levels of PD-1HIGHCD4+ T cells correlate with mature B cells in lymph nodes, and PD-1 blockade in PD-1HIGHCD4+ T and B cell co-cultures significantly inhibits IgG production. In summary, PD-1HIGHCD4+ T cells residing in GC represent a specific TFH subset that contributes to maturation of B cells and IgG production.

Persistent Simian Immunodeficiency Virus Infection Causes Ultimate Depletion of Follicular Th Cells in AIDS

CD4+ T follicular helper (Tfh) cells are critical for the generation of humoral immune responses to pathogenic infections, providing help for B cell development, survival, and affinity maturation of Abs. Although CD4+ Tfh cells are reported to accumulate in HIV or SIV infection, we found that germinal center Tfh cells, defined in this study as CXCR5+PD-1HIGHCD4+ T cells, did not consistently accumulate in chronically SIV-infected rhesus macaques compared with those infected with less pathogenic simian HIV, vaccinated and SIVmac-challenged, or SIVmac-infected Mamu-A*01+ macaques, all of which are associated with some control of virus replication and slower disease progression. Interestingly, CXCR5+PD-1HIGH Tfh cells in lymphoid tissues were eventually depleted in macaques with AIDS compared with the other cohorts. Chronic activation and proliferation of CXCR5+PD-1HIGH Tfh were increased, but PD-L2 expression was downregulated on B cells, possibly resulting in germinal center Tfh cell apoptosis. Together, these findings suggest that changes in CXCR5+PD-1HIGH Tfh cells in lymph nodes correlate with immune control during infection, and their loss or dysregulation contribute to impairment of B cell responses and progression to AIDS.