Envelope glycoproteins • gp120 Is a surface glycoprotein attached to the viral membrane it recognizes specific receptors on the surface of the target cells. • gp41 Mediates fusion of the host and viral membranes and thereby allowing the virus to enter the host cell HIV core • The core of the virus is encased in a cone- shaped structural protein, p24 or capsid • Core consists of: • two single-stranded RNA molecules that form the RNA genome of the virus, and • three enzymatic proteins, reverse transcriptase integrase proteases HIV genome Most retroviruses have three common genes, referred to as the gag-pol-env genes. • Gag encodes the proteins that form the core of the virion. • Env encodes the envelope glycoproteins and • Pol encodes the viral enzymes (reverse transcriptase, proteases and integrase) HIV genome • HIV-1 is however more complex than other retroviruses and contains additional genes, the HIV regulatory genes • These genes code for proteins involved in regulation of gene expression • Some play a role in the HIV disease progression * tat, rev, nef, vif, vpr, and vpu are regulatory genes Cellular receptors for HIV HIV targets the vital defense mechanism Two receptors are required for the virus to gain entry into the host cell: • CD4 receptor • co-receptor (chemokine receptor) Cellular receptors for HIV Cells expressing the CD4 receptor • CD4 T lymphocytes • Monocytes • Macrophages • Langerhans cells in the mucous membranes and skin • Glial cells in the central nervous system (astrocytes, microglia, follicular dendritic cells) • Chromaffin cells in the intestine Other cells • Cervical cells • Retinal cells • Renal epithelia Cellular receptors for HIV Chemokine co-receptors The chemokine receptors are regular receptors for small molecule chemical messengers called chemokines. They mediate fusion between HIV and its target cells. • CCR-5 found on both T lymphocytes and monocytes • CXCR-4 found on T lymphocytes * normally functions as receptors for chemokines in the body. HIV tropism • Co-receptors also determine the cellular tropism of HIV-1 • T lymphocyte cells are the main X4 co- receptor cells • T-tropic HIV-1 strains bind to the X4 co- receptor cells (mainly T cells) • M-tropic HIV-1 strains bind to the R5 co- receptor cells like the macrophages Replication cycle of HIV Replication cycle of HIV Attachment and fusion • Binding of viral gp120 to target host cell CD4 receptor. • gp41-mediated fusion of host and viral membrane then follows leading to internalization of virus. Co-receptor is vital in this stage. • Reverse transcription of viral RNA to viral DNA catalyzed by RT enzyme occurs. Replication cycle of HIV Integration • Viral DNA translocates to the nucleus and is integrated to host cell genome by the enzyme integrase forming proviral DNA. Transcription • Integrated proviral DNA may remain inactive in the nucleus or lead to viral RNA and mRNA transcription on cellular activation. Translation • HIV mRNA is translated to viral proteins Replication cycle of HIV Assembly • Assembly of HIV proteins, enzymes and genomic RNA at the cell plasma membrane forms the viral particle. Viral budding • Budding of immature virion occurs and the core acquires its external envelope • The third viral enzyme, protease cleaves the gag-pol precursor proteins during or just after budding yielding a mature, infectious virion * Potential for therapeutic intervention exists at all the different stages of the life cycle Immunopathogenesis of HIV Normal immune system consists of two arms: • innate (ancient) immune system and • adaptive (acquired) immune system Innate immunity – non-specific, includes: Non-killer cell lymphocytes, dendritic cells, monocytes, macrophages, neutrophils, eosinophils, basophils, tissue mast cells Adaptive immunity – characterized by: antigen- specific responses to a foreign antigen. Divided into: – cellular and – humoral immunity Immunopathogenesis of HIV Adaptive immunity consists of cellular and humoral immunity. • T cells are the main mediators of cellular immunity (CD4 and CD8 T cells). • B cells are the principal effectors of the humoral immunity and lead to specific antibody production once activated. • CD4 T cells are the primary regulatory cells of both cellular and humoral immunity. Immunopathogenesis of HIV • Together, innate and adaptive immunity protect the host from infections by multiple organisms. • HIV entry into the body initiates a complex chain of immunopathogenic mechanisms that lead to a vicious cycle of disease progression. • Finally, profound immunosuppression characteristic of advanced HIV disease occurs. Factors affecting HIV pathogenesis Host factors • cell-mediated immunity (CD4, CD8, B cells, NK cells, macrophages and Langerhans cells) • humoral immunity • genetic factors Viral factors Other factors CD4 WBC Neutrophils Lymphocytes CD4+ CD8+ Cell-mediated immune response CD4 T lymphocytes Depletion and dysfunction of CD4 T cells with persistent HIV infection is central to the pathogenesis of HIV disease. CD4 T cells are the primary regulatory cells of both cellular and humoral immunity. CD8 T lymphocytes Cytotoxic CD8 T lymphocytes (CTL) – HIV–specific CTLs when activated, are cytotoxic to HIV infected CD4 cells and also to uninfected CD4 cells (innocent bystanders). CTLs appear early in primary HIV infection and play a major role in reduction of initial viraemia Suppressor CD8 T lymphocytes – cause a non-cytolytic suppression of HIV replication Cell-mediated immune response B cells Multiple functional abnormalities occur. Polyclonal activation, autoantibody production and hypergammaglobulinaemia Non-Killer cells Involved in the killing of HIV-infected cells Monocytes and macrophages These cells are not easily killed by HIV infection (low cytopathic effect) and are also targets of HIV. Therefore they act as reservoirs of HIV, an obstacle in eradication of HIV with treatment Cell-mediated immune response Dendritic and Langerhan cells First cells to appear at inflamed mucosal sites. Carries the bound virus from inflamed site to the lymphoid organs rich in CD4 cells. Vital in initiating the above T- cell responses. Humoral immune response • HIV-specific antibody production by B cells is important in reducing the viraemia in primary infection stage • Antibodies can be detected within six to twelve weeks by ELISA or Western blot • No clear definite function of these antibodies, some have a protective function while others enhance progression of HIV infection Other mechanisms Autoimmune mechanisms • Production of antibodies against self (auto-antibodies). Several auto-antibodies have been detected in HIV- infected patients. Apoptosis (programmed cell death) • HIV infection may lead to abnormal activation of this process. Anergy: • Immunological unresponsiveness in HIV-infected patients due to inappropriate or wrong signaling of CD4 cells. Lymphoid organs in HIV infection • Plays a major role as a reservoir of HIV. • Viral particles trapped within the FDCs network in lymph node germinal center. • Persistent viral replication occurs, even during the clinical “latency” period facilitating further infection of CD4 T cells by trapped virus. Lymphoid organs in HIV infection • Expanding network of FDCs and chronic, persistent viral replication leads to follicular hyperplasia which presents clinically as enlarged lymph nodes. • In advanced HIV disease, degeneration of FDCs network and involution of lymph node germinal center occurs and trapped HIV virions are released into circulation causing massive ↑ in plasma viremia in late disease Genetic factors in HIV pathogenesis • Some host genetic factors may influence the pathogenesis and course of HIV disease • This is associated with different clinical conditions and different clinical courses of the disease, such as: Genetic factors in HIV pathogenesis • Long-term nonprogressors or slow progressors: CCR5 homozygous mutations are associated with ↓ susceptibility to infection while its heterozygous mutation delays progression of disease. • Rapid progression: HIV-infected patients with homozygous mutations of certain co-receptors (CX3CR1), showed rapid progression to AIDS. Also those with human leukocyte antigen, HLA B35. • Exposed yet uninfected: They remain negative despite repetitive sexual exposure to HIV in high risk environment. In vitro tests have shown this to be due to co-receptor mutations. Viral (HIV) factors • Viral escape from immune response by mutations • Viral attenuation: • E.g. a deletion in the nef gene (a regulatory gene), slows the rate of disease progression • Viral tropism: • Progression from M-tropic to T-tropic viruses associated with ↑ pathogenicity and progression of disease • Viral subtypes – different subtypes (M,N,O) have different virulence and transmissibility Co-factors contributing to HIV pathogenesis Can be endogenous or exogenous factors Cytokines Chemical messengers like TNFα and β, IL-1 and IL-6 (pro-inflammatory) are ↑ in HIV-infected persons and enhance HIV replication. Co-infection Of cells with HIV and other viruses like CMV, HSV type1, EBV, HTLV 1 and others can up-regulate the expression of HIV. Other microbes such as Mycobacterium tuberculosis and Mycoplasma also have a similar effect.
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