When Immunity Kills: The Lessons of SARS-CoV-2 Outbreak.

Since its emergence at the end of 2019, SARS-CoV-2 has spread worldwide at a very rapid pace. While most infected individuals have an asymptomatic or mild disease, a minority, mainly the elderly, develop a severe disease that may lead to a fatal acute respiratory distress syndrome (ARDS). ARDS results from a highly inflammatory immunopathology process that includes systemic manifestations and massive alveolar damages that impair gas exchange. The present review summarizes our current knowledge in the rapidly evolving field of SARS-CoV-2 immunopathology, emphasizing the role of specific T cell responses. Indeed, accumulating evidence suggest that while T-cell response directed against SARS-CoV-2 likely plays a crucial role in virus clearance, it may also participate in the immunopathology process that leads to ARDS.

Follicular CD4 T Cells Tutor CD8 Early Memory Precursors: An Initiatory Journey to the Frontier of B Cell Territory.

The early events of CD8 memory generation remain largely unknown. Here we report that as early as 2 days after antigen priming, very early memory precursors can be identified by their expression of the chemokine receptor CXCR5. These early precursors, which have an effector phenotype, expand and temporarily migrate to the T-B cell zone junction where they interact with follicular CD4+ T cells (Tfh). Remarkably, this interaction with Tfh, hitherto considered as exclusive B cell helpers, is required for CD8 memory precursors to become highly competent memory cells. CD40 and interleukin-21 signaling are involved in the help provided to CXCR5+CD8 memory precursors. This study thus unveils critical early steps in the generation of CD8 memory, identifies CXCR5 as the earliest known marker of CD8 memory precursors, suggests a major helper role for Tfh, and points to possible coordination between the pathways of CD8 and B cell memory generation at the T-B-cell zone junction.

Refractory T-Cell Anergy and Rapidly Fatal Progressive Multifocal Leukoencephalopathy After Prolonged CTLA4 Therapy.

Progressive multifocal leukoencephalopathy (PML) is a deadly demyelinating disease due to central nervous system replication of the human polyomavirus JC virus (JCV) in immunosuppressed patients. The only effective therapeutic approach is to restore anti-JCV T-cell responses. In this study, we describe a case of rapidly fatal PML with JCV T-cell anergy in a renal transplant patient treated with CTLA4-Ig (belatacept, a CD28-B7 costimulation blocker and T-cell anergy inducer). T-cell anergy could not be reversed despite several therapeutic approaches. Progressive multifocal leukoencephalopathy secondary to biotherapy-induced T-cell anergy may thus represent a subset of PML with major resistance to anti-JCV immune recovery.

B cells and progressive multifocal leukoencephalopathy: search for the missing link.

Progressive multifocal leukoencephalopathy (PML) is a deadly demyelinating disease due to JC virus (JCV) replication in the brain. PML classically occurs in patients with severe immunodepression, and cases have recently been linked to therapeutic monoclonal antibodies such as natalizumab and also rituximab, which depletes B cells. B cells appear to play a complex role in the pathogenesis of PML. They may act as a viral reservoir and as a vector for viral dissemination in the central nervous system. Anti-JCV antibody responses appear to have a limited effect on JCV replication in the brain. However, accumulating evidence suggests that B cells may considerably influence T cell responses through their cytokine secretion. This immunomodulatory function of B cells may play an important role in the control of JCV infection and in the pathogenesis of PML, including rituximab-induced PML.

Residual HIV-1 replication may impact immune recovery in patients on first-line lopinavir/ritonavir monotherapy.

BACKGROUND: Antiretroviral combination therapy raises issues of long-term adherence and toxicity. Initial treatment simplification based on single-drug therapy was investigated in the MONARK trial, which compared first-line lopinavir/ritonavir monotherapy (arm A) with first-line lopinavir/ritonavir + zidovudine/lamivudine tritherapy (arm B). The MONARK trial is registered as a randomized trial at clinical trials.gov under identifier NCT 00234923. PATIENTS AND METHODS: Immune recovery was compared in patients with undetectable plasma virus (<50 copies/mL) after 60 weeks of treatment (arm A, n = 21; arm B, n = 13). RESULTS: The week 60 CD4 T cell count and CD4 T cell subset distribution did not differ significantly between the treatment arms. Memory CD4 T cell responses to HIV and recall antigens were better with triple therapy than with monotherapy. The frequencies of activated CD8 T cells and anti-HIV CD8 T cell effector responses were similar in the two arms. However, the repertoire of CD8 T cell effector responses was broader and persistent residual viraemia more frequent (by ultrasensitive PCR) in the monotherapy arm. CONCLUSIONS: While viral control can be achieved with first-line lopinavir/ritonavir monotherapy, the quality of immune recovery is inferior to that obtained with triple therapy, possibly owing to a higher level of residual viral replication. Thus, the benefits of first-line lopinavir/ritonavir monotherapy in terms of toxicity and adherence might be offset by an increased risk of residual viral replication, which may also fuel latent viral reservoirs.

Efficacy of recombinant human interleukin 7 in a patient with severe lymphopenia-related progressive multifocal leukoencephalopathy.

In this study, we report the case of a patient with profound lymphopenia after allogenic bone marrow transplantation who developed severe progressive multifocal leukoencephalopathy. Single-agent recombinant human interleukin-7 therapy was associated with restoration of anti-John Cunningham polyomavirus (JCV) T-cell responses, JCV clearance from cerebrospinal fluid, and a dramatic clinical improvement.

Tissue competence imprinting and tissue residency of CD8 T cells.

T cell immunity is characterized by striking tissue specialization. Tissue-specificity imprinting starts during priming by tissue-derived migratory dendritic cells in the non-random, specialized micro-anatomical area of the draining lymph node and is influenced by constitutive and induced cues from local environment. Besides tissue-specific effectors, memory cells also exhibit a tissue-specificity. Long-lived tissue-resident memory T cells likely play a considerable role in preventing pathogen invasion. Understanding of the mechanisms of tissue specialization of T cells is of major importance for the design of optimal vaccination strategies and therapeutic interventions in tissue/organ-specific inflammatory diseases. The present review summarizes our current knowledge and hypothesis about tissue-specificity imprinting and tissue residency of T cells.

Immunological hallmarks of JC virus replication in multiple sclerosis patients on long-term natalizumab therapy.

Progressive multifocal leukoencephalopathy (PML) is the main adverse effect of natalizumab. Detectable JC virus-specific effector memory T-cell (TEM) responses may indicate ongoing JCV replication. We detected JCV-specific TEM responses in blood of patients with multiple sclerosis (MS) treated with natalizumab, including 2 patients with PML. The frequency of detection of these responses increased with the time on natalizumab. Thus, a subset of MS patients exhibit immunological hallmarks of JCV replication during prolonged natalizumab therapy.

Improved survival of HIV-1-infected patients with progressive multifocal leukoencephalopathy receiving early 5-drug combination antiretroviral therapy.

BACKGROUND: Progressive multifocal leukoencephalopathy (PML), a rare devastating demyelinating disease caused by the polyomavirus JC (JCV), occurs in severely immunocompromised patients, most of whom have advanced-stage HIV infection. Despite combination antiretroviral therapy (cART), 50% of patients die within 6 months of PML onset. We conducted a multicenter, open-label pilot trial evaluating the survival benefit of a five-drug cART designed to accelerate HIV replication decay and JCV-specific immune recovery. METHODS AND FINDINGS: All the patients received an optimized cART with three or more drugs for 12 months, plus the fusion inhibitor enfuvirtide during the first 6 months. The main endpoint was the one-year survival rate. A total of 28 patients were enrolled. At entry, median CD4+ T-cell count was 53 per microliter and 86% of patients had detectable plasma HIV RNA and CSF JCV DNA levels. Seven patients died, all before month 4. The one-year survival estimate was 0.75 (95% confidence interval, 0.61 to 0.93). At month 6, JCV DNA was undetectable in the CSF of 81% of survivors. At month 12, 81% of patients had undetectable plasma HIV RNA, and the median CD4+ T-cell increment was 105 per microliter. In univariate analysis, higher total and naive CD4+ T-cell counts and lower CSF JCV DNA level at baseline were associated with better survival. JCV-specific functional memory CD4+ T-cell responses, based on a proliferation assay, were detected in 4% of patients at baseline and 43% at M12 (P = 0.008). CONCLUSIONS: The early use of five-drug cART after PML diagnosis appears to improve survival. This is associated with recovery of anti-JCV T-cell responses and JCV clearance from CSF. A low CD4+ T-cell count (particularly naive subset) and high JCV DNA copies in CSF at PML diagnosis appear to be risk factors for death. TRIAL REGISTRATION: ClinicalTrials.gov NCT00120367.

Interferon-alpha triggers B cell effector 1 (Be1) commitment.

B-cells can contribute to the pathogenesis of autoimmune diseases not only through auto-antibody secretion but also via cytokine production. Therapeutic depletion of B-cells influences the functions and maintenance of various T-cell subsets. The mechanisms governing the functional heterogeneity of B-cell subsets as cytokine-producing cells are poorly understood. B-cells can differentiate into two functionally polarized effectors, one (B-effector-1-cells) producing a Th-1-like cytokine pattern and the other (Be2) producing a Th-2-like pattern. IL-12 and IFN-γ play a key role in Be1 polarization, but the initial trigger of Be1 commitment is unclear. Type-I-interferons are produced early in the immune response and prime several processes involved in innate and adaptive responses. Here, we report that IFN-α triggers a signaling cascade in resting human naive B-cells, involving STAT4 and T-bet, two key IFN-γ gene imprinting factors. IFN-α primed naive B-cells for IFN-γ production and increased IFN-γ gene responsiveness to IL-12. IFN-γ continues this polarization by re-inducing T-bet and up-regulating IL-12Rß2 expression. IFN-α and IFN-γ therefore pave the way for the action of IL-12. These results point to a coordinated action of IFN-α, IFN-γ and IL-12 in Be1 polarization of naive B-cells, and may provide new insights into the mechanisms by which type-I-interferons favor autoimmunity.

CD25 appears non essential for human peripheral T(reg) maintenance in vivo.

BACKGROUND: IL-2 has been reported to be critical for peripheral T(reg) survival in mouse models. Here, we examined T(reg) maintenance in a series of paediatric liver transplant recipients who received basiliximab, a therapeutic anti-CD25 monoclonal antibody. METHODOLOGY/PRINCIPAL FINDINGS: FoxP3+ CD4 T cells were analyzed by flow cytometry before liver grafting and more than 9 months later. We found that in vivo CD25 blockade did not lead to T(reg) depletion: the proportion of FoxP3+ cells among CD4 T cells and the level of FoxP3 expression were both unchanged. IL-2Rbeta expression was enhanced in FoxP3+ cells both before and after basiliximab treatment, while the level of IL-2Rgamma expression was similar in T(regs) and non-T(regs). No significant change in the weak or absent expression of IL-7Ralpha and IL-15Ralpha expression on FoxP3+ cells was observed. Although the proportion of FoxP3+ cells among CD4 T cells did not vary, food allergies occurred more rapidly after liver grafting in patients who received basiliximab, raising questions as to T(reg) functionality in vivo in the absence of functional CD25. CONCLUSIONS: CD25 appears non essential for human T(reg) peripheral maintenance in vivo. However, our results raise questions as to T(reg) functionality after therapeutic CD25 targeting.

Direct CD4 help provision following interaction of memory CD4 and CD8 T cells with distinct antigen-presenting dendritic cells.

Accumulating evidence suggests that CD4 help is needed at the memory stage to mount effective secondary CD8 T cell responses. In this paper, we report that memory CD4 T cells can provide efficient help to memory CD8 T cells after interaction of the two lymphocytes with distinct dendritic cells. Provision of help to CD8 T cells required direct cell-cell contact and involved both IL-2 and CD40 ligation, within a CD4-CD8 T cell synapse. Thus, following antigenic interaction with APCs, activated memory CD4 and CD8 T cells appear to separate from their respective APCs before meeting each other for help provision, regardless of their Ag specificity. CD4 help for memory CD8 T cells therefore appears to be conditioned primarily not by Ag specificity but by activation status.

Resting regulatory CD4 T cells: a site of HIV persistence in patients on long-term effective antiretroviral therapy.

BACKGROUND: In HIV-infected patients on long-term HAART, virus persistence in resting long-lived CD4 T cells is a major barrier to curing the infection. Cell quiescence, by favouring HIV latency, reduces the risk of recognition and cell destruction by cytotoxic lymphocytes. Several cell-activation-based approaches have been proposed to disrupt cell quiescence and then virus latency, but these approaches have not eradicated the virus. CD4+CD25+ regulatory T cells (Tregs) are a CD4+ T-cell subset with particular activation properties. We investigated the role of these cells in virus persistence in patients on long-term HAART. METHODOLOGY/PRINCIPAL FINDINGS: We found evidence of infection of resting Tregs (HLADR(-)CD69(-)CD25(hi)FoxP3+CD4+ T cells) purified from patients on prolonged HAART. HIV DNA harbouring cells appear more abundant in the Treg subset than in non-Tregs. The half-life of the Treg reservoir was estimated at 20 months. Since Tregs from patients on prolonged HAART showed hyporesponsiveness to cell activation and inhibition of HIV-specific cytotoxic T lymphocyte-related functions upon activation, therapeutics targeting cell quiescence to induce virus expression may not be appropriate for purging the Treg reservoir. CONCLUSIONS: Our results identify Tregs as a particular compartment within the latent reservoir that may require a specific approach for its purging.

Heterospecific CD4 help to rescue CD8 T cell killers.

Help from CD4 T cells may be required for optimal generation and maintenance of memory CD8 T cells and also for optimal Ag reactivation. We examined whether the helper cell and the CD8 killer cell need to have the same Ag specificity for help to be effective during interactions of memory T cells with mature APC. This is important because virus and tumor Ag-specific CD4 T cell responses are selectively impaired in several chronic viral infections and malignancies. We performed studies in vitro and in vivo and found that functional memory CD4 T cells generated from a distinct antigenic source (heterospecific helpers) could provide direct and effective help to memory CD8 T cells. Functional heterospecific memory CD4 T cells could also rescue secondary CD8 T cell responses in an experimental tumor model in which homospecific CD4 help was impaired. This could provide a rationale for immunotherapy strategies designed to bypass impaired homospecific help.

Differential effect of agonistic anti-CD40 on human mature and immature dendritic cells: the Janus face of anti-CD40.

Agonistic monoclonal antibodies to CD40 (CD40 mAbs) have a puzzling dual therapeutic effect in experimental animal models. CD40 mAbs induce tumor regression by potentiating antitumoral T-cell responses, yet they also have immunosuppressive activity in chronic autoimmune inflammatory processes. CD40 mAbs are thought to act on antigen presentation by dendritic cells (DCs) to T cells. DCs can be distinguished as either immature or mature by their phenotype and their ability to generate an effective T-cell response. Here we found that, on human cells, although anti-CD40 led immature DCs to mature and became immunogenic, it also reduced the capacity of lipopolysaccharide (LPS) and tumor necrosis factor alpha (TNF-alpha)-matured DCs to generate a specific CD4 T-cell response. This inhibitory effect was related to rapid and selective apoptosis of mature DCs. Anti-CD40-mediated apoptosis was due to an indirect mechanism involving cooperation with the death domain-associated receptor Fas, leading to activation of Fas-associated death domain protein (FADD) and caspase-8. On human cells, CD40 activation by such agonists could, therefore, trigger immune responses to antigens presented by immature DCs, which are otherwise nonimmunogenic, by inducing maturation. On the other hand, anti-CD40 mAbs, by rapidly inducing apoptosis, may reduce the capacity of inflammatory signal-matured immunogenic DCs to generate an effective T-cell response. These results call for caution in CD40 mAb-based immunotherapy strategies.

In human B cells, IL-12 triggers a cascade of molecular events similar to Th1 commitment.

Two functionally distinct subsets of B cells that produce Th1- and Th2-like patterns of cytokines have recently been identified. Interleukin-12 (IL-12) is a critical immunoregulatory cytokine that promotes Th1 differentiation through activation of signal transducer and activator of transcription 4 (STAT4). IL-12 has been reported to induce interferon gamma (IFN-gamma) production in B cells, but the relevant signaling pathways are poorly documented. Here, in human primary B cells, we found a functional IL-12 receptor (IL-12R) that internalizes following IL-12 binding. IFN-gamma and, to a lesser extent, IL-12 positively regulated the IL-12Rbeta2 subunit but had no effect on IL-12Rbeta1. On examining the effect of IL-12 on STAT4 and T-bet (2 key factors involved in IFN-gamma promoter activation), we found that IL-12 induced the phosphorylation and nuclear translocation of STAT4. IL-12-dependent constitutive STAT4 activation was also observed in the Epstein-Barr virus (EBV)-transformed B-cell line RPMI 8866 that spontaneously produces IL-12. T-bet expression has been shown to be dependent on STAT1. IL-12 had no direct effect on STAT1 activation or T-bet expression in primary B cells. In contrast, IL-12-induced IFN-gamma led to STAT1 activation, strong expression of T-bet, and IFN-gamma expression. IL-12 therefore initiates a cascade of events in B cells, including STAT4 activation, IL-12Rbeta2 up-regulation, IFN-gamma production, and T-bet up-regulation, potentially leading to Th1-like differentiation.

Critical role of JC virus-specific CD4 T-cell responses in preventing progressive multifocal leukoencephalopathy.

BACKGROUND: JC virus (JCV) is ubiquitous among the general population. However, only individuals with severely impaired immunity, mainly AIDS patients, develop progressive multifocal leukoencephalopathy (PML). Here, we examined the role of specific CD4 T cells in the control of JCV infection. METHODS AND DESIGN: JCV-specific CD4 T-cell responses were investigated by assaying peripheral blood mononuclear cell proliferation in response to the purified virus. Four groups of individuals without PML were examined: 14 HIV-seronegative healthy donors and 25 HIV-infected patients without PML, separated into urinary JCV excretors (active infection) and non-excretors, according to JCV PCR on urine. Two groups of patients with PML were also studied: 14 HIV-infected patients with active PML; and 10 PML survivors on effective and prolonged antiretroviral therapy. All of the patients were PCR-positive for JCV in the cerebrospinal fluid at the time of diagnosis of PML. RESULTS: No significant anti-JCV CD4 T-cell proliferation was found in any of the non-excretors tested. All nine healthy donors and seven of the 13 non-PML HIV-infected patients with urinary JCV excretion had positive JCV-specific CD4 T-cell responses. No significant response was found in the 14 patients with active PML, while nine of the 10 PML survivors had positive responses. Restoration of JCV-specific CD4 T-cell responses was associated with JCV clearance from the cerebrospinal fluid. CONCLUSION: JCV-specific CD4 T-cell responses appear to play a critical role in the control of JCV infection, preventing PML development. Such responses can be restored in PML survivors following effective and prolonged antiretroviral therapy.