Role of CD40-Dependent Down-Regulation of CD154 in Impaired Induction of CD154 in CD4+ T Cells from HIV-1-Infected Patients

The Host Autophagy During Toxoplasma Infection

Autophagy is an important homeostatic mechanism, in which lysosomes degrade and recycle cytosolic components. As a key defense mechanism against infections, autophagy is involved in the capture and elimination of intracellular parasites. However, intracellular parasites, such as Toxoplasma gondii, have developed several evasion mechanisms to manipulate the host cell autophagy for their growth and establish a chronic infection. This review provides an insight into the autophagy mechanism used by the host cells in the control of T. gondii and the host exploitation by the parasite. First, we summarize the mechanism of autophagy, xenophagy, and LC3-associated phagocytosis. Then, we illustrate the process of autophagy proteins-mediated T. gondii clearance. Furthermore, we discuss how the parasite blocks and exploits this process for its survival.

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

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.

Cytomegalovirus-specific responses of CD38⁺ memory T cells are skewed towards IFN-γ and dissociated from CD154 in HIV-1 infection

OBJECTIVE

Despite the strong correlation of T-cell CD38 expression with HIV disease progression, evidence linking CD38 expression and dysfunction at the single cell level is scant. Since CD38⁺ memory CD4⁺ T cells, especially those from HIV-infected persons, fail to induce CD154 (CD40L) while responding to a superantigen with interferon (IFN)-γ or interleukin (IL)-2, we aimed to determine if recall responses to cytomegalovirus (CMV) were similarly affected in the CD38⁺ memory CD4⁺ T-cell subpopulation.

DESIGN AND METHODS

Peripheral blood mononuclear cells from HIV+ patients and healthy controls were incubated 14 h with CMV antigens, the superantigen Staphylococcus aureus enterotoxin B or medium, and labeled for identification of central memory (T(CM)) and effector memory (T(EM)) CD4⁺ T cells, and for the intracellular detection of induced CD154, IFN-γ and/or IL-2 by flow cytometry.

RESULTS

Compared with CD38⁻ cells, CD38⁺ T(CM) cells from patients had less CD40L induction after CMV stimulation, and increased IFN-γ response. Patients' CD38⁺ T(EM) cells showed a lower IL-2 response, and tended to have a greater IFN-γ response, in which CD154 induction frequently failed. CMV-specific responses of patients' CD38⁺ T(CM) and T(EM) cells were dominated by IFN-γ, and almost all IL-2⁺ cells co-expressed IFN-γ. IL-2 responses to the polyclonal activator S. aureus enterotoxin B were also significantly less frequent among CD38⁺ T(CM) and T(EM) cells than in CD38⁻ cells.

CONCLUSION

Patients' CD38⁺ memory CD4⁺T-cell responses to CMV favor the effector cytokine IFN-γ over IL-2, in the context of deficient CD154 induction, which may limit co-stimulation, proliferation and survival.

The protein kinase double-stranded RNA-dependent (PKR) enhances protection against disease cause by a non-viral pathogen

PKR is well characterized for its function in antiviral immunity. Using Toxoplasma gondii, we examined if PKR promotes resistance to disease caused by a non-viral pathogen. PKR(-/-) mice infected with T. gondii exhibited higher parasite load and worsened histopathology in the eye and brain compared to wild-type controls. Susceptibility to toxoplasmosis was not due to defective expression of IFN-γ, TNF-α, NOS2 or IL-6 in the retina and brain, differences in IL-10 expression in these organs or to impaired induction of T. gondii-reactive T cells. While macrophages/microglia with defective PKR signaling exhibited unimpaired anti-T. gondii activity in response to IFN-γ/TNF-α, these cells were unable to kill the parasite in response to CD40 stimulation. The TRAF6 binding site of CD40, but not the TRAF2,3 binding sites, was required for PKR phosphorylation in response to CD40 ligation in macrophages. TRAF6 co-immunoprecipitated with PKR upon CD40 ligation. TRAF6-PKR interaction appeared to be indirect, since TRAF6 co-immunoprecipitated with TRAF2 and TRAF2 co-immunoprecipitated with PKR, and deficiency of TRAF2 inhibited TRAF6-PKR co-immunoprecipitation as well as PKR phosphorylation induced by CD40 ligation. PKR was required for stimulation of autophagy, accumulation the autophagy molecule LC3 around the parasite, vacuole-lysosomal fusion and killing of T. gondii in CD40-activated macrophages and microglia. Thus, our findings identified PKR as a mediator of anti-microbial activity and promoter of protection against disease caused by a non-viral pathogen, revealed that PKR is activated by CD40 via TRAF6 and TRAF2, and positioned PKR as a link between CD40-TRAF signaling and stimulation of the autophagy pathway.

Naturally-acquired influenza-specific CD4+ T-cell proliferative responses are impaired in HIV-infected African adults

Background

Seasonal influenza has been associated with greater morbidity and mortality in AIDS patients. Highly-active antiretroviral therapy (HAART) has led to some reduction in influenza-related complications but the nature of naturally-acquired T-cell immunity to influenza virus in an African setting, and how this changes with immune reconstitution following HAART is unknown. We measured influenza-specific CD4⁺ T-cell immunity in unimmunized HIV-infected Malawian adults and then investigated immune reconstitution following HAART.

Methods

Peripheral blood mononuclear cells were isolated from HIV-infected and HIV-uninfected Malawian adults. CFSE proliferation and CD154 expression flow cytometry-based assays were used to measure influenza-specific CD4⁺ T-cell immunity.

Results

We found lower naturally-acquired proliferative influenza-specific CD4⁺ T-cell responses in AIDS patients that was also present in asymptomatic HIV-infected adults with relatively high CD4 counts (>350 cells/µl). Influenza-specific CD4⁺ T-cell immune reconstitution in HIV-infected patients on HAART for 12 months was poor despite a marked reduction in viral load and an increase in CD4 count. This poor immune reconstitution was characterised by a low influenza-specific proliferative CD4⁺ T-cell response and reduced proportions of CD154-expressing influenza-specific CD4⁺ T-cells in peripheral blood.

Conclusion

Our data suggest that asymptomatic HIV-infected adults may also be at risk of influenza-related complications and that HAART alone may not circumvent this risk in AIDS patients. This study highlights the need to identify possible interventions early in HIV infection to reduce the risk of influenza and to intensify influenza surveillance in these susceptible African populations.

Chronic immune activation in HIV-1 infection contributes to reduced interferon alpha production via enhanced CD40:CD40 ligand interaction

Although a signature of increased interferon (IFN-)alpha production is observed in HIV-1 infection, the response of circulating plasmacytoid dendritic cells (PDC) to Toll-like receptor ligand stimulation is substantially impaired. This functional PDC deficit, which we specifically observed in HIV-1 infected individuals with less than 500 CD4+ T cells/µl, is not well understood. We provide evidence that the peripheral IFN-alpha production in HIV-1 infection is actively suppressed by the enhanced interaction of CD40 ligand (CD40L), a member of the tumor necrosis factor family, and its receptor CD40, which are both upregulated upon immune activation. Plasma levels of soluble CD40L were significantly higher in untreated HIV-1 infected individuals (n = 52) than in subjects on long-term antiretroviral therapy (n = 62, p<0.03) and in uninfected control donors (n = 16, p<0.001). Concomitantly, cell-associated CD40L and the expression of the receptor CD40 on the PDC were significantly upregulated in HIV-1 infection (p<0.05). Soluble and cell-associated CD40L inhibited the PDC-derived IFN-alpha production by CpG oligodeoxynucleotides dose-dependently. This suppressive effect was observed at much lower, physiological CD40L concentrations in peripheral blood mononuclear cells (PBMC) of HIV-1 infected individuals compared to controls (p<0.05). The CpG-induced IFN-alpha production in PBMC of HIV-1 infected donors was directly correlated with PDC and CD4+ T cell counts, and inversely correlated with the viral loads (p<0.001). In HIV-1 infected donors with less than 500 CD4+ T cells/µl, the CpG-induced IFN-alpha production was significantly correlated with the percentage of CD40-expressing PDC and the level of CD40 expression on these cells (p<0.05), whereas CD40L plasma levels played a minor role. In addition, low-dose CD40L contributed to the enhanced production of interleukin 6 and 8 in PBMC of HIV-1 infected donors compared to controls. Our data support the conclusion that the chronic immune activation in HIV-1 infection impairs peripheral PDC innate immune responses at least in part via enhanced CD40:CD40L interactions.

Impaired CD4 T cell memory response to Streptococcus pneumoniae precedes CD4 T cell depletion in HIV-infected Malawian adults

OBJECTIVE

Invasive pneumococcal disease (IPD) is a leading cause of morbidity and mortality in HIV-infected African adults. CD4 T cell depletion may partially explain this high disease burden but those with relatively preserved T cell numbers are still at increased risk of IPD. This study evaluated the extent of pneumococcal-specific T cell memory dysfunction in asymptomatic HIV infection early on in the evolution of the disease.

METHODS

Peripheral blood mononuclear cells were isolated from asymptomatic HIV-infected and HIV-uninfected Malawian adults and stained to characterize the underlying degree of CD4 T cell immune activation, senescence and regulation. Pneumococcal-specific T cell proliferation, IFN-γ, IL-17 production and CD154 expression was assessed using flow cytometry and ELISpot.

RESULTS

We find that in asymptomatic HIV-infected Malawian adults, there is considerable immune disruption with an increase in activated and senescent CD4+CD38+PD-1+ and CD4+CD25(high)Foxp3+ Treg cells. In the context of high pneumococcal exposure and therefore immune stimulation, show a failure in pneumococcal-specific memory T cell proliferation, skewing of T cell cytokine production with preservation of interleukin-17 but decreased interferon-gamma responses, and failure of activated T cells to express the co-stimulatory molecule CD154.

CONCLUSION

Asymptomatic HIV-infected Malawian adults show early signs of pneumococcal- specific immune dysregulation with a shift in the balance of CD4 memory, T helper 17 cells and Treg. Together these data offer a mechanistic understanding of how antigen-specific T cell dysfunction occurs prior to T cell depletion and may explain the early susceptibility to IPD in those with relatively preserved CD4 T cell numbers.

Dissociation of CD154 and cytokine expression patterns in CD38+ CD4+ memory T cells in chronic HIV-1 infection

Expression of the activation antigen CD38 on T cells is a strong predictor of the risk of HIV disease progression, but it is not known whether CD38 is a marker or mediator of dysfunction. We examined the relationship between CD38 expression and responses to T-cell receptor stimulation in central memory and effector memory CD4 T cells in HIV-infected persons and in healthy controls. Basal CD38 expression was preserved by blocking golgi transport with brefeldin A. Intracellular expression of interleukin 2, interferon γ, and CD154 was measured after stimulating peripheral blood mononuclear cells with the superantigen staphylococcal enterotoxin B with or without anti-CD28 costimulation. Interferon-γ responses were comparable or increased in stimulated CD38 memory cells, and the interleukin 2 responses of costimulated CD38 central memory cells were decreased in HIV infection. In CD38 cells and especially in CD38 cells of HIV-infected persons, stimulated memory cells more often failed to express CD154 (CD40 ligand) when induced to express cytokine. A dissociated cytokine and CD154 expression by memory CD4 T cells may impair interactions between T cells and antigen-presenting cells, contribute to impaired immunity and help explain the relationship between CD38 expression and disease progression in chronic HIV infection.

The CD40-autophagy pathway is needed for host protection despite IFN-Γ-dependent immunity and CD40 induces autophagy via control of P21 levels

Autophagy degrades pathogens in vitro. The autophagy gene Atg5 has been reported to be required for IFN-γ-dependent host protection in vivo. However, these protective effects occur independently of autophagosome formation. Thus, the in vivo role of classic autophagy in protection conferred by adaptive immunity and how adaptive immunity triggers autophagy are incompletely understood. Employing biochemical, genetic and morphological studies, we found that CD40 upregulates the autophagy molecule Beclin 1 in microglia and triggers killing of Toxoplasma gondii dependent on the autophagy machinery. Infected CD40(-/-) mice failed to upregulate Beclin 1 in microglia/macrophages in vivo. Autophagy-deficient Beclin 1(+/-) mice, mice with deficiency of the autophagy protein Atg7 targeted to microglia/macrophages as well as CD40(-/-) mice exhibited impaired killing of T. gondii and were susceptible to cerebral and ocular toxoplasmosis. Susceptibility to toxoplasmosis occurred despite upregulation of IFN-γ, TNF-α and NOS2, preservation of IFN-γ-induced microglia/macrophage anti-T. gondii activity and the generation of anti-T. gondii T cell immunity. CD40 upregulated Beclin 1 and triggered killing of T. gondii by decreasing protein levels of p21, a molecule that degrades Beclin 1. These studies identified CD40-p21-Beclin 1 as a pathway by which adaptive immunity stimulates autophagy. In addition, they support that autophagy is a mechanism through which CD40-dependent immunity mediates in vivo protection and that the CD40-autophagic machinery is needed for host resistance despite IFN-γ.

Mucosal immunity in resource-limited setting: is the battle ground different?

In many developing countries, populations are under considerable pressure from high bacterial exposure on mucosal surfaces. Immune dysregulation in this setting is multifactorial and is driven by a range of environmental factors, undernutrition and coinfections such as measles, malaria and HIV. Disruption or subversion of respiratory-tract and intestinal epithelial barriers leads to increased invasion by mucosal pathogens and a high frequency of life-threatening bacterial disease. It is our opinion that a process of epithelial barrier dysfunction and immune dysregulation at these mucosal surfaces leads to the much higher rates of pneumonia, meningitis and severe sepsis seen in resource-limited countries.

Combined Env- and Gag-specific T cell responses in relation to programmed death-1 receptor and CD4 T cell loss rates in human immunodeficiency virus-1 infection

Additional progression markers for human immunodeficiency virus (HIV) infection are warranted. In this study we related antigen-specific responses in CD4(+) and CD8(+) T cells to CD38, reflecting chronic immune activation, and to CD4(+) T cell loss rates. Clones transiently expressing CD107a (CD8(+)) or CD154 (CD4(+)) in response to Gag, Env and Nef overlapping peptide pools were identified, along with their expression of the inhibitory programmed death-1 receptor (PD-1) in fresh peripheral blood mononuclear cells (PBMC) from 31 patients off antiretroviral treatment (ART). HIV-specific CD8(+) T cell responses dominated over CD4(+) T cell responses, and among CD8(+) responses, Gag and Nef responses were higher than Env-responses (P < 0.01). PD-1 on CD8(+) HIV-specific subsets was higher than CMV-specific CD8(+) cells (P < 0.01), whereas PD-1 on HIV-specific CD4(+) cells was similar to PD-1 on CMV-specific CD4(+) cells. Gag and Env CD8(+) responses correlated oppositely to the CD4 loss rate. Env/Gag CD8(+) response ratios, independently of PD-1 levels, correlated more strongly to CD4 change rates (r = -0.50 to -0.77, P < 0.01) than the total number of Gag-specific CD8(+) cells (r = 0.44-0.85, P < or = 0.02). The Env/Gag ratio performed better than CD38 and HIV-RNA in logistic regression analysis predicting CD4 change rate as a measure of progression. In conclusion, HIV-specific CD8(+)CD107a(+) Env/Gag response ratio was a stronger predictor for progression than CD38 and HIV-RNA. The Env/Gag ratio may reflect the balance between possibly beneficial (Gag) and detrimental (Env) CD8(+) T cell responses and should be explored further as a progression marker.

CD40, autophagy and Toxoplasma gondii

Toxoplasma gondii represents a pathogen that survives within host cells by preventing the endosomal-lysosomal compartments from fusing with the parasitophorous vacuoles. The dogma had been that the non-fusogenic nature of these vacuoles is irreversible. Recent studies revealed that this dogma is not correct. Cell-mediated immunity through CD40 re-routes the parasitophorous vacuoles to the lysosomal compartment by a process called autophagy. Autophagosome formation around the parasitophorous vacuole results in killing of the T. gondii. CD40-induced autophagy likely contributes to resistance against T. gondii particularly in neural tissue.

Autophagy in immunity against Toxoplasma gondii

A decisive outcome during host-pathogen interaction is governed by whether pathogen-containing vacuoles fuse with lysosomes. Fusion with lysosomes typically kills microbes. Toxoplasma gondii represents a classical example of an intracellular pathogen that survives within host cells by preventing the endosomal-lysosomal compartments from fusing with the vacuoles that contain the pathogen. Thus, T. gondii provides an excellent model to determine if the immune system can target a pathogen for lysosomal degradation. CD40, a major regulator of cell-mediated immunity, activates macrophages to kill T. gondii through a process that requires recruitment of autophagosomes around the parasitophorous vacuole, leading to lysosomal degradation of the parasite. These studies demonstrate that cell-mediated immunity can activate autophagy to kill a pathogen. CD40-induced autophagy likely contributes to resistance against T. gondii, particularly in neural tissues, the main sites affected by this pathogen.

CD40 and the immune response to parasitic infections

The interaction between CD40 and CD154 regulates many aspects of cellular and humoral immunity. The CD40-CD154 pathway is important for resistance against a variety of parasites. Studies done with these pathogens have provided important insight into the various mechanisms by which this pathway enhances host protection, mechanisms by which pathogens subvert CD40 signaling, conditions in which the CD40-CD154 pathway promotes disease and on modulation of this pathway for immunotherapy.

Selected Toll-like receptor ligands and viruses promote helper-independent cytotoxic T cell priming by upregulating CD40L on dendritic cells

CD40L (CD154) on CD4(+) T cells has been shown to license dendritic cells (DCs) via CD40 to prime cytotoxic T lymphocyte (CTL) responses. We found that the converse (CD40L on DCs) was also important. Anti-CD40L treatment decreased endogenous CTL responses to both ovalbumin and influenza infection even in the absence of CD4(+) T cells. DCs expressed CD40L upon stimulation with agonists to Toll-like receptor 3 (TLR3) and TLR9. Moreover, influenza infection, which stimulates CTLs without help, upregulated CD40L on DCs, but herpes simplex infection, which elicits CTLs through help, did not. CD40L-deficient (Cd40lg(-/-)) DCs are suboptimal both in vivo in bone marrow chimera experiments and in vitro in mixed lymphocyte reactions. In contrast, Cd40lg(-/-) CD8(+) T cells killed as effectively as wild-type cells. Thus, CD40L upregulation on DCs promoted optimal priming of CD8(+) T cells without CD4(+) T cells, providing a mechanism by which pathogens may elicit helper-independent CTL immunity.