Human Immunodeficiency Virus Lecture

Human immunodeficiency virus (HIV) HIV is the cause of acquired immune deficiency syndrome (AIDS)

Characteristics of the virus • Is a single stranded, diploid RNA, enveloped virus of the lentivirus subfamily of retroviruses. • The reverse transcriptase makes copies of DNA from the viral RNA.

Characteristics of the virus • Outer envelope contains a lipid matrix within which specific viral glycoproteins are imbedded. • These knob-like structures responsible for binding to target cell.

Human immunodeficiency virus

Structural Genes • Group Specific Antigen (Gag)- encodes core protein (p24) & matrix protein (p17). • Envelope (Env)- encodes gp160, cleaved into gp120 & gp41. • Polymerase (Pol)- encodes reverse transcriptase, protease & integrase enzyme.

Group Specific Antigen (Gag) • Gag gene encodes for nucelocapsid & matrix protein of virus. • Neocleocapsid protein (p24) used in serological test. • p17 lies between protein core and envelope.

Envelope (Env) • Envelope (Env) gene codes for envelope protein gp160 will eventually be cleaved by proteases to gp120 and gp41. • The gp120 binds specifically to CD4 receptor on host cell, mutates rapidly & results in antigenic variation. • Gp41 causes fusion of the virus to the cell membrane & virus particle enters cell.

Polymerase (Pol) • Polymerase (Pol) codes for reverse transcriptase, protease & integrase enzyme. • Located in the core, close to nucleic acids. • Responsible for conversion of viral RNA into DNA, integration of DNA into host cell DNA and cleavage of protein precursors.

Important enzymes • Reverse Transcriptase (RT) – converts RNA genome into a DNA copy • Integrase (IN) – inserts the DNA copy into host genome • Protease (PR) – Cuts and processes immature HIV proteins into mature, functioning proteins

Methods of transmission • Sexually, presence of STD increases transmission. • Exposure to infected blood or blood products. • Sharing contaminated needles. • Transplantation of infected tissues or organs. • Mother to fetus, perinatal transmission variable, dependent on viral load and mother’s CD 4 count.

Methods of transmission • HIV has been found in saliva and tears in very low quantities from AIDS patients, but has never been shown to result in transmission of HIV. • Researchers at CDC and elsewhere have shown no evidence of HIV transmission through insects.

Pathogenesis • First step, HIV attaches by gp120 to the CD4 antigen found on a variety of cells● helper T cells ● macrophages ● monocytes ● B cells ● microglial brain cells ● T cells infected later on.

Pathogenesis • In early phase HIV infection, viruses are M-tropic. • Their gp120 is able to bind to CD4 molecule and chemokine receptors called CCR5 found on macrophages

Pathogenesis • In late most of the viruses are T-tropic, • Having gp120 capable of binding to CD4 and CXCR4 found on T4-lymphocytes.

Clinical findings • Clinical features occur in three stages: • Acute stage • Latent stage • Late, immunodeficiency stage

Acute symptoms • Mononucleosis-like, cold or flu-like symptoms may occur 2 to 4 weeks after infection. – Fever, sore throat, Lymphadenopathy, Rash, headache, Fatigue or no symptoms may be present

Window period • Antibody appears 3-4 wks after infection with HIV, before antibody is detected the patient is serologically “false negative” for HIV infection. • Patient is highly infectious, • Lab diagnosis during window period• P24 antigen assay • Viral Culture • PCR

Clinical Latency Period • After initial viremia, a viral set point (viral load) occurs. • Patient is asymptomatic at this period. • Though the patient is asymptomatic viral replication occurs.

Late stage • CD4 count gradually declines from its normal value of 500-1200. • Once CD4 count drops below 200/μl, infected person is considered to have advanced HIV disease & at risk for opportunistic infections and malignancy.

AIDS • In late stage neurological problem e.g. dementia and neuropathy develops – Most deaths occur with CD4 counts below 50.

Opportunistic Infections • Respiratory system

● Central/peripheral Nervous system – Pneumocystis Carinii - Cytomegolavirus Pneumonia. - Toxoplasmosis – Tuberculosis - Cryptococcosis – Kaposi's Sarcoma - Non Hodgkin's lymphoma • Gastro-intestinal system - Varicella Zoster – Cryptosporidiosis - Herpes simplex – Candida ● Skin – Cytomegolavirus - Herpes simple – Isosporiasis - Kaposi's sarcoma – Kaposi's Sarcoma - Varicella Zoster

Oral Candidiasis (thrush)

Oral Hairy Leukoplakia

Kaposi’s sarcoma (KS) • Kaposi’s sarcoma is a rare cancer of the blood vessels that is associated with HIV. It manifests as bluish-red oval-shaped patches that may eventually become thickened. Lesions may appear singly or in clusters.

Laboratory Diagnosis of HIV Infection • • • • •

Antibody detection Antigen detection Detection of viral nucleic acid Viral isolation Indirect predictors of HIV infection

Three types of tests

(i) Screening tests - ELISA and simple/rapid tests. (ii) Confirmatory- Western Blot assay. (iii) Nucleic acid based tests- Polymerase chain reaction (PCR).

ELISA Test • Antibodies detected in ELISA against: p24, gp120, gp160 and gp41 antigen. • ELISAs are for screening only • False positives occur in alcoholism, syphilis, and immunoproliferative diseases.

Other Screening Tests • Agglutination tests: Latex particles, Gelatin particles and • Comb-dipstick based systems • Dot-Blot Test: A rapid, cost-effective. • Most have sensitivities and specificities of 99% and 98% respectively

Rapid Tests • Advantages: – quicker to perform – do not require specialised equipment or trained personnel

Western Blot • Most popular confirmatory test. • Antibodies to p24 and p55 appear earliest but decrease or become undetectable. • Antibodies to gp31, gp41, gp 120, and gp160 appear later but are present throughout all stages of the disease.

Indirect immunofluorescence •

Can be used to detect both virus and antibody to it.

Viral Load Tests • Viral load is the quantity of HIV-RNA in the blood. • Viral load tests measure the amount of HIV-RNA in one milliliter of blood.

Issues in Testing  HIV antibody testing and

counseling should always be voluntary and confidential.