ANN IST SUPER SANITÀ 2010 | VOL. 46, NO. 1: 5-14 5 DOI: 10.4415/ANN_10_01_02 HIV virology and pathogenetic mechanisms ENCE of infection: a brief overview I EXPER (a) (b) (b) (a) L Emanuele Fanales-Belasio , Mariangela Raimondo , Barbara Suligoi and Stefano Buttò A C (a)Centro Nazionale AIDS, Istituto Superiore di Sanità, Rome, Italy I N (b)Centro Operativo AIDS, Dipartimento di Malattie Infettive, Parassitarie ed Immunomediate, I CL Istituto Superiore di Sanità, Rome, Italy TO NG I Summary. Studies on HIV virology and pathogenesis address the complex mechanisms that result TEST in the HIV infection of the cell and destruction of the immune system. These studies are focused L on both the structure and the replication characteristics of HIV and on the interaction of the A M virus with the host. Continuous updating of knowledge on structure, variability and replication I N of HIV, as well as the characteristics of the host immune response, are essential to refine virologi- A cal and immunological mechanisms associated with the viral infection and allow us to identify key molecules in the virus life cycle that can be important for the design of new diagnostic assays FROM and specific antiviral drugs and vaccines. In this article we review the characteristics of molecular structure, replication and pathogenesis of HIV, with a particular focus on those aspects that are RCH important for the design of diagnostic assays. A Key words: HIV, virus replication, antigenic variation, virulence. ESE R Riassunto (La virologia dell’HIV e i meccanismi patogenetici dell’infezione: una breve panoramica). Gli studi sulla virologia e la patogenesi dell’HIV sono importanti per comprendere i complessi meccanismi che regolano l’infezione della cellula da parte del virus e la distruzione del sistema immunitario. Questi studi si focalizzano sulle caratteristiche della struttura e della replicazione di HIV e sui meccanismi di interazione con l’ospite. Infatti, l’aggiornamento continuo di questi aspetti della biologia dell’HIV è fondamentale, poiché può portare all’identificazione di nuove molecole che giocano un ruolo chiave nella patogenesi del virus e che possono indirizzare nuovi campi di ricerca per la generazione di farmaci, vaccini e nuovi saggi diagnostici. Questo breve articolo riassume brevemente le attuali conoscenze sulle caratteristiche molecolari, del ciclo repli- cativo e della patogenesi dell’HIV, dando particolare rilievo ad aspetti che risultano importanti per l’allestimento di saggi diagnostici. Parole chiave: HIV, replicazione del virus, variazione antigenica, virulenza. INTRODUCTION stricted to some regions of Western and Central HIV virology and pathogenetic mechanisms of Africa. HIV is a genetically related member of infection are continuously being investigated. A de- the Lentivirus genus of the Retroviridae family. tailed understanding of HIV structure and how it Infections with lentiviruses typically show a chronic establishes infection and causes AIDS are crucial course of the disease, with a long period of clinical not only to identify and develop new effective drugs latency, persistent viral replication and involvement and vaccines, but also to define strategies for the lab- of the central nervous system. oratory diagnosis of HIV infection. HIV testing is The retrovirus genome is composed of two iden- a critical step that allows to control HIV spreading tical copies of single-stranded RNA molecules [1] in the population. Laboratory diagnostic strategies and is characterized by the presence of structural have to be continuously revised according to new genes gag, pol, env. HIV-1 and HIV-2 viruses differ discoveries on the replication characteristics and the in the organisation of their genome, although the pathogenetic mechanisms of HIV infection. basic structure (i.e. the presence of the three struc- tural genes, gag, pol and env) is the same one as for all retroviruses. In fact, in addition to having these STRUCTURE AND REPLICATION OF HIV three genes, the HIV-1 and HIV-2 genomes present Human Immunodeficiency Virus (HIV) isolates a complex combination of other regulatory/acces- are currently grouped into two types, HIV-type 1 sory genes (Figure 1). (HIV-1) and HIV-type 2 (HIV-2). The worldwide Both viruses potentially cause AIDS, though dis- main agent of AIDS is HIV-1, while HIV-2 is re- ease of the central nervous system may be more fre- Address for correspondence: Stefano Buttò, Centro Nazionale AIDS, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy. E-mail: [email protected]. 6 Emanuele Fanales-Belasio, Mariangela Raimondo, Barbara Suligoi, et al. � ��� ��� ��� ��� ��� ��� ��� ��� ��� ��� ��� � ��� ��� ��� ��� ��� ��� ��� ��� ��� ��� Fig. 1 | Organisation of the HIV genome. ��� A) HIV-1 genome; B) HIV-2 genome. quent in HIV-2 infection [2]. In addition, HIV-2 ap- spanning gp41 glycoproteins bound together. The pears less virulent than HIV-1 and infection course binding between gp120 and gp41 is not covalent takes longer to progress to AIDS [3]. and therefore the gp120 may be shed spontaneously The structure of the HIV particle is similar for within the local environment and detected in the se- both HIV-1 and HIV-2 and is shown in Figure 2. rum, as well as within the lymphatic tissue of HIV- Similarly to other retroviruses, the gag gene encodes infected patients. During the process of budding the structural proteins of the core (p24, p7, p6) and from the infected cell (see later), the virus may also matrix (p17) and the env gene encodes the viral en- incorporate into its membrane different proteins velope glycoproteins gp120 and gp41, which recog- from the host cell membrane, such as HLA class I nize cell surface receptors. The pol gene encodes for and II proteins, or adhesion proteins such as ICAM-1 enzymes crucial for viral replication, which are the that may facilitate adhesion to other target cells. A reverse transcriptase that converts viral RNA into matrix protein (p17) is anchored to the inside of the DNA, the integrase that incorporates the viral DNA viral lipoprotein membrane. Virus membrane and into host chromosomal DNA (the provirus) and the the matrix protein include the capsid composed of protease that cleaves large Gag and Pol protein pre- polymers of the core antigen (p24). The capsid con- cursors into their components. tains two copies of HIV RNA combined with a nu- HIV viral particles have a diameter of 100 nm and cleoprotein and the enzymes reverse transcriptase, are surrounded by a lipoprotein-rich membrane. integrase and protease (reviewed in ref. [4]). Each viral particle membrane includes glycopro- HIV viruses are characterised by other accessory/ tein heterodimer complexes composed of trimers of regulatory genes that play key roles in modulating the external surface gp120 and the transmembrane virus replication (reviewed in ref. [5]). Among these, Envelope gp120 gp41 Lipidic membrane Matrix p17 Capsid p24 Reverse transcriptase Fig. 2 | Structure of the HIV-1 ssRNA particle. ssRNA: single strand RNA. HIV VIRUS AND PATHOGENICITY 7 Infecting HIV New HIV particle 1. Binding 6. Budding fusion and entry 6 1 CD4+ cell 2 Viral ssRNA 2. Uncoating 5 5. Protein synthesis and RT assembly 3. Uncoating transcription 3 RNA/DNA mRNA Viral dsDNA Provirus 4. Integration of 4 viral DNA into Cell nucleus host chromosome Fig. 3 | HIV replication cycle. RT: reverse transcriptase; dsDNA: double strand DNA. the tat gene encodes for a protein (Tat) that is ex- surface to allow virus budding in the late stages of pressed very early after infection and promotes the the virus replication cycle. expression of HIV genes. The Rev protein, coded by the rev gene, ensures the export from nucleus to cytoplasm of the correctly processed messenger THE HIV REPLICATION CYCLE and genomic RNA. The function of the other ac- The HIV replication cycle is schematically shown cessory HIV proteins is less well understood; it is in Figure 3. It can be summarised in six steps; 1) believed that the Vpr protein is involved in the ar- binding and entry; 2) uncoating; 3) reverse tran- rest of the cell cycle. This protein also enables the scription; 4) provirus integration; 5) virus protein reverse transcribed DNA to gain access to the nu- synthesis and assembly and 6) budding. cleus in non-dividing cells such as macrophages, a The entry pathway of HIV-1 and HIV-2 can be function that is performed by Vpx in HIV-2. Vpu is divided into three major events: virus binding to a protein necessary for the correct release of virus the cell, activation and fusion (Figure 4). The viral particle, whereas the vif gene codes for a small pro- envelope trimeric complex, composed of the het- tein (Vif) that enhances the infectiveness of progeny erodimer proteins gp120 and gp41, is essential for virus particles. Finally, the Nef protein has multiple virus recognition and entry into target cells. Indeed, functions including cellular signal transduction and the gp41 subunit contains a fusogenic hydrophobic the down regulation of the CD4 receptor on the cell peptide at its amino terminus, which is essential for A B M-tropic HIV T-tropic HIV T-tropic HIV M-tropic HIV CXCR-4 CD4 CCR-5 CD4 Fig. 4 | HIV tropism. M-tropic HIV: monocyte/macrophage- CD4+ cell CD4+ cell tropic HIV; T-tropic HIV: T-lymphocyte-tropic HIV. 8 Emanuele Fanales-Belasio, Mariangela Raimondo, Barbara Suligoi, et al. fusion of the viral and cellular membranes [6]. HIV pronged attachment of the virus, which, in turn, gp120 binds a 58 kDa monomeric glycoprotein, de- allows the N-terminal fusion peptide gp41 to pen- signed as CD4, which is expressed on the cell surface etrate the cell membrane. The HR1 and HR2 repeat of about 60% of circulating T-lymphocytes, on T-cell sequences in gp41 interact, causing the collapse of precursors within the bone marrow and thymus, on the extracellular portion of gp41 into a hairpin.