Retroviruses possess an enzyme, reverse transcriptase,that can transcribe ssRNA into double-stranded DNA.This activity is reflected in the designation“retroviruses.”Integration of the DNA thus derived from the viralgenome in the host-cell genome is a precondition forviral replication.
PARTICULAR PROPERTIES OFRETROVIRUSESViruses: have2 identical RNAEnzyme reverse transcriptasearehighly-changeable (because reverse transcriptase is not under control and a lot of mistakes arepresent )
SUBFAMILY OF RETROVIRIDAE1. Oncornaviridae – are capable of oncogenic cell transformation, -rna- contain RNA2. Spumaviridae – CPE - syncytia look like foam, spume3. Lentiviridae – (from lent, long time)
ONCOVIRUSES INCLUDE: Oncoviruses D – causative agents of monkey’smammary cancer Oncoviruses B – causative agents of cancer of mice,guinea pigs’s mammary Oncoviruses C – causative agents of mammalia andbirds leukemia and sarcomasThe term-complex “leukemia-sarcoma” is used foranimals oncogenic viruses (cat, hamsters, monkey,rats, rabbits,. ) HTL viruses types I and II - human T-cellleukemia virusthe bovine leukemia virus.
SPUMAVIRUSESonly occur in animals (cattle, monkey,hamsters, cats…),two of which are (probably) from humans
LENTIVIRIDAEHIV (human immunodeficiency virus) 1 and 2,maedi-virus (pneumonia),visnavirus (encephalo-myelitis) in sheep, virusesaffecting goats and horses,animal immune deficiency viruses.
HUMAN IMMUNE DEFICIENCYVIRUS (HIV)Is a causative agent of AIDS (acquired immune deficiencysyndrome). This term used since 1982.Was developed by R. Hallo (USA) and L.Montanie (France) in 1983- HIV I.Two types of HIV are known: HIV I,II (1987).The types of HIV can be distinguished genetically and antigenically.HIV-1 is the cause of the current worldwide pandemic while HIV-2 is found in west Africa but rarely elsewhere.HIV-2, which is transmitted in the same ways as HIV-1, causesAIDS much more slowly than HIV-1 but otherwise clinically thediseases are very similar. Both HIV-1 and HIV-2 are thought tohave arisen from simian immunodeficiency virus (SIV).HIV-2 is closely related to the SIV found in west Africa.
MAIN TYPES HIVHIV-I HIV-IIvirulent high lesswidespread Europe, N. America West Africagenome Homological in 42 %Near to MIV (monkey)SIV (simian)Antigenic structure(mol. weight in kdt )gp 120gp 41matrix protein 17p24gp 125gp 36matrix protein 17p26
SUB-GROUPS AND SUB-TYPESThere are three sub-groups ofHIV-1,M (main or major),N (new) andO (outlier).Type O HIV-1 is mostly foundin Cameroon and Gabonwhile the rare N sub-group isalso found in Cameroon. It isvery likely that SIV infectedhumans on separate occasionsto give rise to the three sub-groups.Based on nucleotide sequenceanalyses of the env and gaggenes, it has been found thatthere are also at least tendifferent HIV-1 subtypes
In some countries, mosaics (recombinants) between differentsubtypes have been found. These arise when two different subtypesinfect a person at the same time and recombination occurs. Theformer subtype I is a circulating recombinant form (or CRF) that isa recombinant of subtypes A, G, H and K.There is some evidence from laboratory studies that differentHIV-1 subtypes can be transmitted by different routes.For example, type B found in western countries, may betransmitted more effectively by homosexual intercourse and viablood (as in intra-venous drug use) whereas types C and E may betransmitted more via a heterosexual route. This is because types Cand E replicate better in Langerhans cells found in the mucosa ofthe cervix, vagina and penis while type B replicates better in therectal mucosa. It also appears that type E is more readilytransmitted between sexual partners than type B.Subtype D seems to be more virulent than subtype A, that isinfected persons progress to overt disease more rapidly. In additionsubtypes D and C seem to be transmitted more effectively frommother to child than subtype A.
CULTIVATION HIVin tissue culture :H-9 (clone TCD4-lymphocytes from leukemiaTCD4-limphocytes)in monolayer of astrocytesCPE: syncytial formationLab. Animals - Chimpanzee
Electron micrograph ofHIV - Cone-shaped coresare sectioned in variousorientations.Viral genomic RNA islocated in the electron-dense wide end of core.CDC/Dr. Edwin P. Ewing,Jr. email@example.com
OTHER INTERNAL PROTEINSThese are encoded by the pol (polymerase) gene. They areenzymes that participate in integration and replication:Reverse transcriptase - copies RNA genome into doublestranded DNAIntegrase - integrates the double stranded DNA into thehost cell chromosomeProtease - cleaves the pol and gag-encoded polyproteins
CELLS THAT ARE INFECTED BY HIVHIV lyses CD4+ T4 cells specifically, causing profound immuno-suppression.Other cells tend to harbor and replicate the virus without lysis or, inthe case of dendritic cells, they may concentrate virus at the cellsurface with little or no replication of the virus.CD4+ T4 helper cells - HIV leads to disease as a result of thedepletion of CD4+ T4 helper cells and the consequent inability tofight opportunistic infections.Natural Killer cells -These are also CD4+ T cells and interact withdendritic cells. In addition to CD4 antigen, they express the co-receptor CCR5 and are thus infected by those HIV strains thatrequire CCR5 for entry into the cell.
CD8+ Killer T cells- These cells express low levels of CD4 antigenwhen they are activated and appear to be infected in smallnumbers by HIV in the later stages of disease. Naive CD8 cells donot express CD4 antigen and do not appear to be infected(although they do express the co-receptors).Macrophages- Monocytes/macrophages express CD4 antigen(although in much lower amounts that T4 cells) and are infectedby HIV. They may provide an important reservoir for the viruswithin the host and may be especially important in HAART-treated patientsCells of the nervous system- HIV infects oligodendrocytes,astrocytes, neurones, glial cells and brain macrophages.Macrophage-tropic forms are found in the cerebro-spinal fluid.Dendritic cells- Follicular dendritic cells (FDCs) are important inthe biology of HIV. These are antigen-presenting cells thatprocess antigen and present peptides to T cells. They are notreadily infected by HIV, though they can be productively infected
ENTRY OF HIV VIA THE MUCOSAL ROUTEAND TRANSIT VIA DENDRITIC CELLS TOTHE LYMPH NODES When HIV enters the bodyvia the mucosal route(epithelia of the vagina, penisor rectum), it is bound byFDCs that migrate to thelymph nodes; here the FDCspresent HIV to T4 cells,which become infected.
STAGES OF THE HIV REPLICATIONCYCLEBinding to surface receptorsFusion of viral and host cell membranesUncoating of the nucleocapsidReverse transcription of RNA to DNAIntegration of DNA provirus into the host genomeReplication (Host DNA polymerase)Transcription (Host RNA polymerase II)RNA splicing in the host cell nucleus (host and viral factors)Translation of viral proteinsProteolysis of viral polyproteins (Host protease in the Golgi Body processes gp160,Viral protease processes GAG,POL)Glycosylation of gp160Phosphorylation of viral proteinsFatty acylation of GAGAssembly of the virus at the host cell membraneBuddingMaturation of the virus particle after buddingThe stages in italics are likely targets since they are specific to the virus
1. ATTACHMENT OF HIV TO A CD4+ CELL.The outer domain ofgp120 binds to the CD4antigen. This leads to aconformational change ingp120 and a co-receptorbinding site is exposed.This region of gp120 bindsto the chemokinereceptor. Binding to thechemokine receptor allowsanother conformationalchange to occur so thatregions of the gp41 HIVprotein interact to form afusion domain that allowsthe viral and cellmembrane to fuse. As aresult the viral core enters
Chemokine receptors are involved, in association with CD4 antigen,in infection by HIV (left). The chemokine can block attachment ofthe virus to its receptors (middle). Mutations in the chemokinereceptor can lead to resistance to HIV infection (right)
CHEMOKINES AREsmall secreted proteins that are chemotactic for cellsin the immune system such as leukocytes which move up thegradient of chemokine secreted by another cell;thus, they control the temporal and spatial positioning of leukocytesduring an immune response.Chemokines are divided into two groups according to a conserveddicysteine motif that they contain.These are the C-C group and the C-X-C group. They bind to the cellsurface via receptor molecules that are integral membraneproteins that span the plasma membrane seven times (seventransmembrane receptors). The receptors are named for the typeof cytokine that they bind (CCR- or CXCR-).
2. ENRTY INTO CELL: PH INDEPENDENTFUSION WITH PLASMA MEMBRANENo pH-dependent conformational change in a viral membraneprotein is necessary for fusion between the viral membrane and themembrane of the cell to be infected. Thus, no entry into endosomesor lysosomes is required.As with herpes virus, this sort of fusion of a virus with the plasmamembrane is associated with fusions of infected cells to formsyncytia. Syncytium formation is also a characteristic of HIVinfection.This has profound significance for spread of infection between cellswithout any free virus. This means that virus may spread from cellto cell so that immune system circulatory antibodies cannot haveany effect (problem for vaccine). Not only will a vaccine have to beable to destroy the virus, it will also have to be able to destroyinfected cells. Gp41 is the fusogen. Syncytia are most often seen inbrain.
3. REVERSE TRANSCRIPTION AND INTEGRATIONThis is similar to other retroviruses. HIV uses reverse transcriptaseimported during infection as part of the virus. The nucleocapsidenters the cytoplasm and reverse transcription occurs within thenucleocapsid.After uncoating and entry into the nucleus, both linear andcircular forms of the viral DNA are found. Linear double strandviral DNA is inserted into the host cell chromosomes using the viralintegrase protein (translated from the pol gene).4. After integration, viral RNA is transcribed by host RNApolymerase II.
FORMATION OF POLYPROTEINS AND THEIRCLEAVAGEAssembly of new virus takes place at the membrane of the host cell. Three typesof protein make up the virion. These are the membrane protein complex (Gp120and Gp41 - originally derived from Gp160) plus two internal precursorproteins, the Gag polyprotein and the Gag/Pol polyprotein (the latter is theresult of a frame shift that allows the ribosome to continue translation from theGag gene into the Pol gene)The proteins aggregate at the cell membrane and the membrane pinches off .The larger internal precursor (Gag-Pol) draws two strands of the positivestrand RNA into the nascent virion and the protease (part of the Gag-Polprotein) cuts itself free. The protease completes the cleavage of Gag-Pol toliberate other enzymes (reverse transcriptase, integrase and more protease).The protease also cleaves the remainder of Gag-Pol and the smaller Gag intostructural proteins. p24, p7 and p6 form the bullet-shaped core while p24underlies the membrane. The Gag and Gag/Pol fusion proteins are made inratio of about 20:1.This specific viral protease is vital as the viral proteins are not functional unlessseparated. This specificity makes the protease a good candidate inhibition byanti-HIV drugs (see appendix 3 and anti-viral chemotherapy sections). Gag/Poland Gag are attached to the viral membrane via a fatty acid that is covalently
PATHOGENESISSelective affection of cells with CD4 receptor (T-helpers,macrophages, monocytes…)Virus is not detected by immune system because: viralgenome is integrated into DNA of host cell and ishighly-changeableVirus has high speed of replication (5000 copies per 5 min)Gp 120 (separately from virion) in blood mark cells withCD4 receptors and as result these cells will be destroy byT-killersViruses can penetrate into cells by intracellular canals (soantibody will not find virus)Virus also infect precursor of T-lymphocytes in tymus andborn marrow (a new T cells will be not differentiated)
Scanning electron micrograph of HIV-1 budding from cultured lymphocyte. Multipleround bumps on cell surface represent sites of assembly and budding of virions (CDC)
DURING THE COURSE OF INFECTION, THEREIS A PROFOUND LOSS OF THE SPECIFICIMMUNE RESPONSE TO HIV BECAUSE:responding CD4+ cells become infected. Thus, there is clonaldeletion leading to tolerance. The cells that proliferate torespond to the virus are infected and killed by itepitope variation can lead to escape of HIV from the immuneresponseactivated T cells are susceptible to apoptosis. Spontaneousapoptosis of uninfected CD4+ and CD8+ T cells occurs in HIV-infected patients. Also there appears to be specific apoptosis ofHIV-specific CD8+ cellsthe number of follicular dendritic cells falls over time, resulting indiminished capacity to stimulate CD4+ cellsT-killers find and destroy all cells with molecules gp120(uninfected to)the synthesis of interleukins is destroyedthe functions of complement and macrophages are depressed
ONSET OF DISEASE - AIDSThe period of clinical latency varies in length from 1 to 2years to more than 15 years.Onset of AIDS is rare in less than 3 years except inchildren. But, eventually, the virus can no longer becontrolled as helper CD4+ (T4) cells are destroyed.Ironically, the killer cells needed to control HIV alsodamage the helper T cells that they need to functionefficiently. With the lack of CD4+ cells, new cytotoxic Tcell responses cannot occur as helper cells are lacking andsuch new responses are required as the virus mutates. Asthe T4 cells fall below 200 per cu mm, virus titers riserapidly and immune activity drops precipitously. It is theloss of immune competence that enables normally benignopportunistic parasites such as viruses, fungi or protozoa
Once AIDS develops, patients rarely survive more than two yearswithout chemotherapeutic intervention. There is considerablevariability at this stage. Some patients with clinical AIDS dosurvive for several years while others who appear relativelyhealthy can suddenly succumb to a major opportunistic infection.It is the onset of HIV-associated cancers and opportunisticinfections that defines AIDS proper. At this stage, also,syncytium-inducing HIV appear in many (about half) AIDSpatients. These are more CD4+ cell tropic than the initiallyinfecting HIV and this contributes to the rapid loss of CD4+ cellsin later stages of the disease.It should be noted that a phase of HIV infection, AIDS-relatedcomplex (ARC), used to be defined. This is now little used. It is thephase of the disease that lacks the neoplasms and opportunisticinfections that are the definition of AIDS. Patients at this stage ofthe disease show weight loss and fatigue together with fungalinfections of the mouth, finger and toe nails.
SYMPTOMS OF HIV DISEASEIncubation period - of some week - to some yearsThe first symptoms - fever, headache, sore throat, muscle aches,enlarged lymph nodes and generalized rash (typically subsideabruptly within a few weeks). Many patients are asymptomatic orexperience mild symptoms attributed to «the flu». For themajority of patients the first concern is persistent enlargement oflymph nodes.The disease slowly advances, most people remain unaware of theirinfection until the development of AIDS.AIDS represent the end stage of HIV disease (patients become highlysusceptible to infections) - EB virus, pneumonia, meningitis,diarrhea,.. Progressive dementia, weight loss, appearance ofmalignant tumors often accompany the AIDS, (Kaposi’s sarcoma)
METHODS OF LABORATORYDIAGNOSISMaterial - blood, sperma, other tissueMolecular biology – PCR – detection viral genomeSerology: 1) detection viral antigens2) detection antiviral antibodiesby ELISA, Immune blotting (more sensitive)Microscope examination,Isolation and identification of virus – for research(not used in ordinary laboratory)
Steps of PCR1. Isolation DNA (RNA)2. Amplificationtemperature cyclesamount of fragments ofthe new copies DNA (RNA)3. Detection in agarosegele
EPIDEMIOLOGYSource - infected personroutes of transmissions - transffusion (blood trasfusion,shered drug injection equipment with infected person,tatoo or pirsing with not steril instrument); sexualcontact; from mather to fetus (during pregnency, atbirth,...)susceptible - general
HIV/AIDS AROUND THE WORLDThe overwhelming majority of people with HIV, some95% of the global total, live in the developing world. Theproportion is set to grow even further as infection ratescontinue to rise in countries where poverty, poor healthcare systems and limited resources for prevention and carefuel the spread of the virus.The chart on the right shows the distribution of peopleliving with HIV around the world, according to 2007 data.
GLOBAL HIV/AIDS ESTIMATES, END OF 2007THE LATEST STATISTICS ON THE WORLD EPIDEMIC OF AIDS &HIV WERE PUBLISHED BY UNAIDS/WHO IN JULY 2008, ANDREFER TO THE END OF 2007.Estimate RangePeople living with HIV/AIDS in 2007 33.0 million 30.3-36.1 millionAdults living with HIV/AIDS in 2007 30.8 million 28.2-34.0 millionWomen living with HIV/AIDS in 2007 15.5 million 14.2-16.9 millionChildren living with HIV/AIDS in 2007 2.0 million 1.9-2.3 millionPeople newly infected with HIV in 2007 2.7 million 2.2-3.2 millionChildren newly infected with HIV in 2007 0.37 million 0.33-0.41 millionAIDS deaths in 2007 2.0 million 1.8-2.3 millionChild AIDS deaths in 2007 0.27 million 0.25-0.29 million
More than 25 million people have died of AIDS since 1981.Africa has 11.6 million AIDS orphans.At the end of 2007, women accounted for 50% of all adultsliving with HIV worldwide, and for 59% in sub-SaharanAfrica.Young people (under 25 years old) account for half of allnew HIV infections worldwide.In developing and transitional countries, 9.7 million peopleare in immediate need of life-saving AIDS drugs; ofthese, only 2.99 million (31%) are receiving the drugs.
REGIONAL STATISTICS FOR HIV & AIDS, ENDOF 2007RegionAdults &childrenliving withHIV/AIDSAdults &childrennewly infectedAdultprevalence*Deaths ofadults &childrenSub-Saharan Africa 22.0 million 1.9 million 5.0% 1.5 millionNorth Africa & Middle East 380,000 40,000 0.3% 27,000Asia 5 million 380,000 0.3% 380,000Oceania 74,000 13,000 0.4% 1,000Latin America 1.7 million 140,000 0.5% 63,000Caribbean 230,000 20,000 1.1% 14,000Eastern Europe & CentralAsia1.5 million 110,000 0.8% 58,000North America, Western &Central Europe2.0 million 81,000 0.4% 31,000Global Total 33.0 million 2.7 million 0.8% 2.0 million
During 2007 more than two and a half million adults andchildren became infected with HIV (HumanImmunodeficiency Virus), the virus that causes AIDS.By the end of the year, an estimated 33 million peopleworldwide were living with HIV/AIDS. The year alsosaw two million deaths from AIDS, despite recentimprovements in access to antiretroviral treatment.NotesAdults are defined as men and women aged 15 or above,unless specified otherwise.Children orphaned by AIDS are defined as people agedunder 18 who are alive and have lost one or both parentsto AIDS.
SummuryBefore the appearance of HIV, AIDS-like syndromes were rare, today they arecommon in HIV-infected peopleThe main risk factors for AIDS are sexual contact, transfusions, IV drugs,hemophilia. These have existed for years but only after the appearance of HIV,has AIDS been observed in these populationsInfection by HIV is the ONLY factor that predicts that a person will developAIDSNumerous serosurveys show that AIDS is common in populations with anti-HIVantibodies but is rare in populations with a low seroprevalence of anti-HIVantibodiesNew-born infants with no behavioral risks develop AIDS if infected as a result ofthe mother being HIV-infectedSince the appearance of HIV, mortality has increased dramatically amonghemophiliacsStudies of transfusion-acquired AIDS has repeatedly led to discovery of HIV inrecipient as well as donorSex partners of HIV-infected hemophiliacs and transfusion patients acquire thevirus and AIDS without other risk factorsHIV infects and kills CD4+ T cells in vitro and in vivo
PREVENTIONNon speciphicCaution in handling blood. Safe sex. Screen blood and organdonors.