HIV (Human Immunodeficiency Virus) PDF

HUMAN IMMUNODEFICIENCY VIRUS (HIV) Dr/ Reem Abdelrahman Lecturer of microbiology and immunology Faculty of medicineHelwan university  Objectives: Recognize Structure of HIV virus. O List its Modes of transmission. Understand HIV Pathogenesis....

HUMAN IMMUNODEFICIENCY VIRUS (HIV) Dr/ Reem Abdelrahman Lecturer of microbiology and immunology Faculty of medicineHelwan university  Objectives: Recognize Structure of HIV virus. O List its Modes of transmission. Understand HIV Pathogenesis. Describe HIV Clinical picture Recognize HIV Diagnosis Identify Treatment and Prevention of HIV 2  Introduction Retroviruses are RNA viruses. Contain reverse transcriptase enzyme and replicate in a unique manner. They cause tumors in several species of animals. There are two important human retroviruses in the Retroviridae family. 1- Human immunodeficiency virus (HIV. 2- Human T cell lymphotropic virus (HTLV). 3 HUMAN IMMUNODEFICIENCY VIRUS (HIV) HIV viruses are non-oncogenic cytolytic slow retroviruses that cause the acquired immunodeficiency syndrome. HIV in humans originated from cross-species infections by simian viruses in rural Africa. Infected individuals may remain asymptomatic, but they are infected for life. A high percentage of carriers develop fatal illness after several years. Virus structure The virion is a 100 nm spherical particle Two identical molecules of single stranded positive sense RNA. composed of a cylindrical internal core of proteins (p 24 and 17). Surrounded by envelope contains virus specific glycoproteins (gp 120 and gp 41). The core contains the viral genome which is a diploid single stranded positive sense RNA and the enzymes; reverse transcriptase (RT), integrase and a protease. The genome contains 9 genes. Three of the genes; ü gag gene encodes the core proteins. ü pol gene encodes the enzymes. ü env gene encodes the envelope proteins. ü tat and rev are replication regulatory genes. ü Four accessory genes.  Surface gp 120 glycoproteins gp 41 Env gene (Envelope) Internal P 24 structural P 17 Gag gene proteins (Core) Reverse transcriptase Proteins Integrase Pol gene (Enzymes) Protease Surface glycoproteins (encoded by env gene): Surface gp 160: glycoproteins Encoded by env gene Precursor glycoprotein cleaved to form gp 120 & gp 41 (type specific envelope glycoproteins. gp 160 Gp 120 (protrudes from the surface): Interacts with CD4 receptors on the cell surface. Antibodies against it neutralizes infectivity. gp 120 The gene encodes gp 120 mutates rapidly that limits vaccine production. Gp 41 (embedded in the envelope): gp 41 Mediates fusion between virus envelope and cell membrane of infected cells.  Internal structural proteins (core proteins) (encoded by gag gene) P 24: The most important core protein is p24 (the capsid), It appears in the serum early after infection and is a serologic marker for virus replication. P 17: matrix protein surrounds p24.  Proteins encoded by pol gene: Reverse transcriptase. Integrase. Polymerase. HIV replication cycle: Attachment: The initial step is binding of the gpl20 to the CD4 protein and the coreceptors on the cell surface, then the gp41 mediates fusion of the viral envelope with the cell membrane, and the virion enters the cell. Uncoating: Intracellular synthesis and assembly: RT transcribes the genome RNA into double stranded DNA (provirus), which integrates into the host cell DNA by the action of the integrase enzyme. Viral mRNA is transcribed from the proviral DNA and transported to the cytoplasm, where it is translated into large polyproteins. These are cleaved by the protease enzyme to produce the different viral structures and enzymes. budding from the cell.  Mode of transmission Blood Sexual Perinatal Initial infection of T cells Initial infection in local dendritic cells then spread to T cells in local lymph nodes Attachment CD4 CXCR CCR5 Acute stage cytotoxic CD8 lymphocytes (CTLs) and antibodies dramatically reduce HIV levels Latent (viral escape) Suppression of immune mechanisms Viral mechanisms T helper- cytotoxic cell deletion-APCs-Humoral immunity Provirus-rapid Mutation-Follicular dendritic cells (reservoir) Late Massive T cell destruction  Mode of transmission: Blood Sexual Perinatal 16 17  Mode of transmission: 1-Sexual contact: Both homosexual and heterosexuals are at high risk. The virus is present in semen and vaginal secretions. 2-Parenteral transmission by transfusion of blood or its products has been greatly reduced by screening donated blood for HIV antibodies. However, there is a window period early in infection when the blood of an infected person can contain HIV but no antibodies. Pricking by contaminated needles or syringes can transmit infection to those at risk e.g. drug abusers and health care workers. 3-Mother to fetus, transplacental, or to the newborn during delivery by exposure to blood in the birth canal or from the milk on breast feeding. 18  Initial infection of T cells. The initial infection of the genital tract occurs in dendritic cells that line the mucosa, these migrate to local lymph nodes where helper T cells become infected. Pathogenesis and immunity: HIV attacks CD4 helper T cells leading to their depletion. The CD4 molecules on these cells are the major receptors for attachment of the virus glycoprotein, gpl20. A protein coreceptor CXCR4 (chemokine receptor) on T cells is required for infection of these cells. Macrophages, monocytes and dendritic cells express CD4 surface molecules and are infected by the virus. A coreceptor CCR5 on these cells is required for their infection. The virus survives in these cells which act as a reservoir and can transport it to other organs e.g. brain and lungs.  During the acute stage cytotoxic CD8 lymphocytes (CTLs) and antibodies dramatically reduce HIV levels, then, the virus escapes from the immune system and establishes chronic latent infection (clinical latency) through: I-Viral mechanisms: a- Latent infection of host cells as a provirus. b- Rapid genetic mutation. c- Trapping of infectious virus in lymphoid tissues in and on the surface of follicular dendritic cells, which act as reservoirs. II-Suppression of immune mechanisms which are more evident during late stages of infection: a-Deletion of CD4 T cells, b-Deletion of HIV-specific CTL clones, c- Dysfunction of CTLs due to; decreased production of IL-2, and decreased expression of MHC-I by the action of viral genes, leading to decreased recognition of virus infected cells by CTLs. d- Impaired functions of APC. e- Interference with humoral response which are T cell dependent. The massive destruction of T cells during the late stages is explained as follows: a- It is found that infected CD4 T cells express high levels of HIV envelope glycoproteins on their surface, this leads to fusion with CD4 molecules on neighboring uninfected cells. This is followed by lysis of large numbers of fused cells and rapid depletion of CD4 helper T cells leading to marked suppression of the immune response, b- Strains of HIV found in the late stages are more virulent and cytopathic than the strains of virus found early in infection and may act as superantigens.  HIV clinical findings Acute Latent Late (AIDS)  Clinical findings The clinical picture of HIV infection can be divided into three stages: an early, acute stage; a middle, latent stage; and a late, immunodeficiency stage. The acute stage begins 2-4 weeks after infection with a mononucleosis-like picture of generalized lymphadenopathy, fever, sore throat and a maculopapular rash. This stage occurs in 87% of those infected, however, it resolves spontaneously in 2 weeks. Antibodies to HIV appear 3-4 weeks after infection. There is rapid replication of the virus, which disseminates to various organs specially lymph nodes. An immune response, by cytotoxic T cells and humoral immunity, controls the infection. 26 A latent stage (clinical latency) for up to 10 years follows. The patient is asymptomatic, viraemia is low or absent, a large amount of HIV is being produced and is trapped by the follicular dendritic cells in lymph nodes where they remain sequestrated. A syndrome called AIDS related complex (ARC) may occur late in the latent stage, characterized by fatigue, wasting, fever, chronic diarrhea, and persistent lymphadenopathy. ARC often progresses to AIDS. 27 The late stage of HIV infections is AIDS. There is decline in the number of CD4 cells to below 400/ul and an increase in the frequency of opportunistic infections. The most characteristic are Kaposi's sarcoma and Pneumocystis jiroveci pneumonia. Others are viral infections e.g. CMV, disseminated herpes simplex and herpes zoster infections; fungal infections such as C. albicans, cryptococcal meningitis; bacterial infections such as, M avium intracellular, M. tuberculosis; protozoal infections such as toxoplasmosis and cancer e.g.. non-hodgkin's lymphoma and cervical cancer. Many patients have neurologic problems e.g. dementia and neuropathy. 28 Pediatric AIDS; Newborns that acquire HIV infection from the mothers -if untreated- have a poor prognosis. They usually present with clinical symptoms by 2 years of age; death follows in another 2 years from opportunistic infections. High levels of plasma HIV load indicate that the infant is at risk of rapid progression of the disease. 29 Acute Confirmed AIDS Viral load (PCR): P24 antigens Follow treatment.Predict AIDS production Opportunistic infection Antibodies CD4 cell count: {appears late) Predict AIDS Very low CD4 PCR Virus isolation cell count 30 Laboratory Diagnosis: 1- Decreased CD4 cells count and inversion of the CD4/CD8 ratio. 2- Detection of HIV antibodies which appear 6-12 weeks after infection. Antibodies are detected by ELISA. A positive test must be repeated and confirmed by the Western blot technique which is more specific and it detects antibodies against viral core protein p24 or envelope glycoproteins gp 41, gp 120 or gpl60 and others. Simple rapid tests are available. 3- Detection of viral nucleic acid in clinical samples by PCR based assays. Quantitative PCR is used to determine plasma HIV RNA i.e. viral load, which helps in monitoring the effectiveness of antiviral therapies. 4-Virus isolation from lymphocytes, bone marrow or plasma in special laboratories. 5- Detection of viral antigens i.e. p24 by ELISA, which may be the only marker in the window period early in infection when antibodies are not detected. Donated blood should be tested for this antigen to assure its safety. However, the antigen often becomes undetectable after antibodies appear (because the p24 protein is complexed with p24 antibodies) but may reappear late in the course of infection, indicating a poor prognosis. 31 During the acute stage of HIV infection, antibody testing is usually negative. Diagnosis is made by viral isolation or by detection of viral RNA by PCR assay or by detection of p24 antigen. Diagnosis during this stage is important as it allows early treatment and helps in preventing infection of sexual partners. Patients with more than 10,000 copies of viral RNA/ml plasma are more likely to progress to AIDS than those with less numbers. Diagnosis of pediatric AIDS is best established by detecting viral nucleic acids by PCR, viral isolation or detection of p24 antigen. Assays for IgM antibodies are remarkably insensitive because of its low concentration. Commercial kits are not available. 32  Treatment: A combination of 2 nucleoside analogues (reverse transcriptase inhibitors) in addition to a protease inhibitor is the treatment of choice for acute HIV infections. Other regimens are used. Therapy with combination of antiretroviral drugs is referred to as HAART or "highly active antiretroviral therapy". These regimens: lower virus load to undetectable levels, delay emergence of resistant mutants, allow the recovery of the immune responses to opportunistic pathogens prolong patient survival. However, the virus persists and reappears when HAART is discontinued. So combination therapy has turned HIV infection into a chronic treatable disease. New drugs are, integrase inhibitors and chemokine receptor (CCR5) antagonists that inhibit viral entry into cells 33  Prevention and control Preventing Vaccine Pre exposure Post exposure perinatal unavailability prophylaxis prophylaxis transmission 34 Prevention and control: No vaccine is available and several are under trial. ?????????? However, vaccine production for HIV is difficult due to several factors; ü Rapid mutation of the virus. ü Absence of an appropriate animal model as well as the unclear understanding of which host immune responses are protective. Prevention consists of taking measures to avoid exposure to the virus, e.g. sex education, not sharing needles, screening of donated blood for HIV. Contaminated wounds should be washed with soap and water. Blood spills on surfaces are disinfected by chlorine 1% for 10 min. 35  Post-exposure chemoprophylaxis after a needle-stick injury by AZT, lamivudine and a protease inhibitor for 2-4 weeks prevents infection. Measures to reduce transmission of HIV infection from infected mothers to fetus or newborn include: ü Screening of pregnant mothers for HIV, ü and those infected should receive AZT or nevirapine during pregnancy and intravenous AZT during delivery. ü Neonates should receive the same drugs. ü Delivery by cesarean section is recommended. ü Infected mothers should not breast feed their infants. 36 Thank you