Kinetics of CXCR4 and CCR5 up-regulation and human immunodeficiency virus expansion after antigenic stimulation of primary CD4+ T lymphocytes

CD4+ T Cell Subsets and Pathways to HIV Latency

Latent infection of CD4⁺ T cells is the main barrier to eradicating HIV-1 infection from infected patients. The cellular and molecular mechanisms involved in the establishment and maintenance of latent infection are directly linked to the transcriptional program of the different CD4⁺ T cell subsets targeted by the virus. In this review, we provide an overview of how T cell activation, T cell differentiation into functional subsets, and the mode of initial viral infection influence HIV proviral transcription and entry into latency.

Interaction between HIV and Mycobacterium tuberculosis: HIV-1-induced CD4 T-cell depletion and the development of active tuberculosis

PURPOSE OF REVIEW

HIV infection is the main driver of the HIV/tuberculosis (TB) syndemic in southern Africa since the early 1990s, when HIV infection rates started to increase exponentially and TB incidence rates quadruplet simultaneously. Here, we discuss pathogenic mechanisms of HIV-induced CD4 T-cell depletion and their potential impact on immune control of Mycobacterium tuberculosis.

RECENT FINDINGS

Depletion of effector memory CD4 T cells from the air-tissue interphase, their dysfunctional regeneration and the preferential depletion of MTB-specific CD4 T cells from circulation and from the air-tissue interphase might be key factors for the increased susceptibility to develop active TB after HIV infection.

SUMMARY

Early initiation of antiretroviral therapy or the development of an efficacious HIV vaccine would be the best options to reduce morbidity and mortality associated with the HIV/TB syndemic.

Pathogen-specific T cell depletion and reactivation of opportunistic pathogens in HIV infection

During HIV infection, it is unclear why different opportunistic pathogens cause disease at different CD4 T cell count thresholds. Early work has shown that CD4 T cell depletion is influenced both by cellular activation status and expression of viral entry receptors. More recently, functional characteristics of the CD4 T cells, such as cytokine and chemokine production, have also been shown to influence cellular susceptibility to HIV. Here, we examine how functional differences in pathogen-specific CD4 T cells could lead to their differential loss during HIV infection. This may have implications for when different opportunistic infections occur, and a better understanding of the mechanisms for functional imprinting of antigen-specific T cells may lead to improvements in design of vaccines against HIV and opportunistic pathogens.

Antigen stimulation induces HIV envelope gp120-specific CD4(+) T cells to secrete CCR5 ligands and suppress HIV infection

CD4(+) T cells are critical for effective immune responses against HIV, but they are also the main cell type targeted by the virus. To investigate the key factors that could protect these cells from infection, we evaluated the capacity of HIV gp120-specific human CD4(+) T cells to produce chemokines that inhibit HIV and determined their contribution in suppressing infection in the cells. Antigen stimulation of the CD4(+) T cells elicited production of high amounts of CCR5 chemokines MIP-1alpha (CCL3), MIP-1beta (CCL4), and RANTES (CCL5). Production of these CCR5 ligands was more readily and reproducibly detected than that of IFN-gamma or IL-2. Importantly, in association with secretion of the CCR5 ligands, antigen stimulation made these CD4(+) T cells more resistant to CCR5-tropic HIV-1. Conversely, in the absence of antigen stimulation, the cells were readily infected by the virus, and after infection, their capacity to produce MIP-1beta and IFN-gamma rapidly declined. Thus, vaccines that trigger HIV-specific CD4(+) T cells to elicit robust and rapid production of anti-viral chemokines would be advantageous. Such responses would protect virus-specific CD4(+) T cells from HIV infection and preserve their critical functions in mounting and maintaining long-lasting immunity against the virus.

Immunophenotypic alterations in acute and early HIV infection

To understand the extent of immune dysregulation in primary HIV infection (PHI) and the impact of antiretroviral therapy (ART) on restoring these abnormalities, we longitudinally evaluated 52 subjects (Acute-Treated (AT); Early-Treated (ET); Early Untreated (EU)) for markers of activation, proliferation, and function on T cells. ET and AT patients differed by 0.54 log viral load (VL) at baseline but did not differ thereafter by more than 0.34 log10 VL. AT subjects had higher CD8(+) T cell counts and expression of markers indicative of CD8(+) T cell activation (CD38), and proliferation (Ki67), at baseline, than ET subjects but were not different 48 weeks post-ART. Although acute PHI is marked by higher level of immune activation than early PHI, virologic and immunologic responses were similar post-ART, suggesting that the extent of immunologic recovery is not negatively impacted by a delay of treatment beyond the acute stage of disease.

Human immunodeficiency virus 1 Nef protein downmodulates the ligands of the activating receptor NKG2D and inhibits natural killer cell-mediated cytotoxicity

Natural killer (NK) cells are a major component of the host innate immune defence against various pathogens. Several viruses, including Human immunodeficiency virus 1 (HIV-1), have developed strategies to evade the NK-cell response. This study was designed to evaluate whether HIV-1 could interfere with the expression of NK cell-activating ligands, specifically the human leukocyte antigen (HLA)-I-like MICA and ULBP molecules that bind NKG2D, an activating receptor expressed by all NK cells. Results show that the HIV-1 Nef protein downmodulates cell-surface expression of MICA, ULBP1 and ULBP2, with a stronger effect on the latter molecule. The activity on MICA and ULBP2 is well conserved in Nef protein variants derived from HIV-1-infected patients. In HIV-1-infected cells, cell-surface expression of NKG2D ligands increased to a higher extent with a Nef-deficient virus compared with wild-type virus. Mutational analysis of Nef showed that NKG2D ligand downmodulation has structural requirements that differ from those of other reported Nef activities, including HLA-I downmodulation. Finally, data demonstrate that Nef expression has functional consequences on NK-cell recognition, causing a decreased susceptibility to NK cell-mediated lysis. These findings provide a novel insight into the mechanisms evolved by HIV-1 to escape from the NK-cell response.

HIV-1 gp120/V3-derived epitopes promote activation-induced cell death to superantigen-stimulated CD4+/CD45RO+ T cells

The third hypervirable (V3) domain of the HIV-1 envelope glycoprotein gp120 has been implicated in HIV pathogenesis via co-receptor usage of chemokine receptors CCR5 and CXCR4. As the protagonist cell populations in the asymptomatic phase of HIV-1 infection are infected macrophages and effector/memory (CD45RO+) CD4+ T cells that express CCR5, we established an in vitro model using human primary monocyte-derived macrophages and lymphocytes to investigate the role of V3 in affecting antigen presentation. We used staphylococcal enterotoxin A (SEA) as a superantigen at a low concentration of 1ng/ml, to activate naïve CD4+ T cells. Exposure of cells to SEA and lipoV3-liposomes increased the percentage of CD4+/CD45RO+/CCR5+ T cell population as compared to cells treated with SEA and plain liposomes. A consequent decrease of the percentage of CD4+/CD45RO+/CXCR4+ subset was observed. The V3-mediated activation was competitively inhibited by soluble V3-derived peptides with higher cationic charge. V3 enhanced also apoptosis as demonstrated by flow cytometry and intracellular calcium ion assays. These results reinforce the postulation that V3 alters the antigen presentation function itself, independent of specific antigens, thus leading to an enhanced activation-induced cell death (AICD) of responding T cells.

CXCR4 and CCR5 regulation and expression patterns on T- and monocyte-macrophage cell lineages: implications for susceptibility to infection by HIV-1

Chemokine receptor expression may vary dramatically among cell subsets. Therefore, the stage of differentiation and the lineage of CD4 cells may profoundly affect their susceptibility to infection by human immunodeficiency virus type 1 (HIV-1). However, the mechanisms of coreceptor competition for association with HIV-1 glycoproteins remain unknown. Here, we propose mathematical models that address the interdependence of the concentrations of CD4 and CCR5 for efficient infection by M-tropic HIV-1 as well as additional complications originated by coreceptor competition caused by posttranslational modifications that positively or negatively affect the coreceptor ability to form complexes with CD4 and/or HIV-1 envelope. Furthermore, since CCR5 and CXCR4 expression on human leukocytes designate these cells as HIV-1 potential targets, the expression of the major HIV-1 coreceptors are also dynamically modeled/quantified as function of the stage of cell differentiation. Results show that although coreceptor competition degree has limited influence on R5 strain infectivity, the infectivity of CXCR4-using isolates strongly depends on the CD4 expression, according to the coreceptor competition model proposed in Lee et al. [J. Virol. 74(11) (2000) 5016]. Understanding the role of in vivo alterations in CD4, CCR5 and CXCR4 densities on HIV-1 cell entry may help the development of optimal control strategies for AIDS pathogenesis.

Increased CCR5 and CXCR4 expression in Ethiopians living in Israel: environmental and constitutive factors

HIV coreceptors play a major role in determining susceptibility and HIV cell tropism. The present work studied whether the high expression of these coreceptors found on lymphocytes and monocytes of Ethiopian immigrants to Israel (ETH) is the result of environmental and/or constitutive genetic factors. The study of 26 ETH shortly after their arrival to Israel (new ETH), 22 ETH in Israel over 7 years (old ETH), and 20 Caucasian Israelis (non-ETH) enabled us to address this issue. The new ETH had elevated levels of activated HLA-DR+CD4+ and CD38+CD8+ cells in comparison with both old ETH and non-ETH groups (P < 0.01), most probably related to chronic helminthic infections. Surface CCR5 expression, i.e., the percentage of CCR5+ cells and the number of CCR5 molecules/cell, was higher (2- to 3- and 8- to 31-fold, respectively) in activated than in nonactivated CD4+ cells, in all groups. However, CCR5 expression, in both activated and nonactivated CD4+ cells, was higher in both ETH groups than in the non-ETH group. CXCR4 expression was higher in nonactivated CD4+ cells in all groups and was also higher in both ETH groups, in both activated and nonactivated CD4+ cells, than in the non-ETH group. These findings suggest that constitutive factors, in addition to immune activation caused by environmental factors, account for the elevated expression of CCR5 and CXCR4 on CD4+ cells of ETH. This increased HIV coreceptor expression may make ETH more susceptible to HIV infection and may account in part for the rapid spread of AIDS in Ethiopia and the rest of Africa as well.