Human Immunodeficiency Virus
HIV was discovered to be the etiological agent of AIDS in 1983. Since its initial discovery, HIV has been experimentally demonstrated to exist in two distinct forms: HIV-1 and HIV-2, the former of which is notably more virulent and widespread than the latter. While HIV-1 accounts for more than 99% of worldwide infection, HIV-2 accounts for a mere 0.11% and is mainly confined to regions in India and Western Africa. HIV-1 is further subdivided into three phylogenetically distinct and highly divergent subgroups: M (major), O (outlier), and N (new: non-M, non-O). HIV-1 group M is the subgroup responsible for the global HIV/AIDS pandemic, accounting for nearly 99.6% of all human HIV infections that are distributed across all continents. Thus, as a result, HIV-1 group M is the subject of much research (Kandathil et al).
HIV-1 group M contains a great amount of genetic diversity between viral species, resulting in the groupís further division into eleven subtypes designated A-K (Travers et al). Such subdivision of group M into subtypes was wholly based on the sequence diversity of envelope (abbreviated env), one of the major genes within all retroviruses, as well as HIV-1, group M viruses, as this is the gene locus harboring the vast majority of the generated genetic diversity.
Since the products of env, gp120 and gp41, are collectively the first contact of HIV with the immune system, they are by definition under the most pressure to acquire differences via generation of genetic diversity in order to better evade the detection and surveillance of the host immune system. The high error rate of viral enzyme reverse transcriptase, recombination events that occur during viral replication, polymorphisms that are present within the human population, and pressure from the host immune response all heavily contribute to the genetic diversity present within env. Data shows that less than 10% of differences exist between env sequences in viruses of the same subtype, and more than 15% difference between env sequences of viruses of distinct subtypes (Bjorndal et al). The overall genetic differences amongst most subtypes are due to these differences in env sequences and account for the variability in infectivity, transmissibility, course of HIV infection, vaccine efficacy, diagnosis, and treatment (Tscherning et al). A previously underappreciated area of research, the genetic diversity within env across viral subtypes of HIV-1, specifically the diversity present within gp120 and within the V3 loop, may have important implications for treatment of HIV-1 with antiretroviral drugs.
Process of Viral Entry into Target Cell
The env gene of HIV is transcribed, translated, and glycosylated to form a single polyprotein product, glycoprotein 160 (gp160), which is subsequently cleaved into glycoprotein 120 (gp120), an N-terminal extracellular protein, and gp41, a C-terminal transmembrane protein. These two glycoproteins non-covalently associate at the membrane surface: each subunit of the trimeric, normally hidden gp41 associates in a non-covalent manner with a trimeric spikes of gp120, thereby anchoring gp120 in the viral membrane as the outermost receptor of HIV (Kieny et al).
HIV uses its envelope protein complex, gp120 and gp41, in order to selectively bind with high affinity to its cellular receptor protein, CD4, located on the exterior of T cells. This binding between gp120/gp41 and CD4 is responsible for mediating the process of viral and cellular membrane fusion and ultimately facilitating the entry of HIV into the host cell (Lu et al). While CD4 is necessary for viral entry into host target cells, it is not sufficient (Kieny et al). Lu et al demonstrated that although the interaction between gp120/gp140 and CD4 induces conformational changes in gp120 that increase the exposure of the third variable loop of env (V3 loop), it is not sufficient in allowing for the membrane fusion reaction to occur. The V3 loop is a specific amino acid sequence within gp120 that is highly subject to genetic mutation and is critical to viral entry. Furthermore, studies have shown that expression of CD4 in non-human cell lines did not render these cells susceptible to infection by HIV, suggesting that another factor, namely a co-receptor, plays a critical role in mediating membrane fusion and viral entry into the host cell (Lu et al). Thus, it was demonstrated that the binding of gp120/gp41 to CD4 and the subsequent induction of a conformational change of gp120 that increased the exposure of the V3 loop requires the interaction of co-receptors CCR5 or CXCR4 with the V3 loop in order for viral entry to occur (Tscherning et al). This binding of gp120/gp41 to both CD4 and chemokine receptor CCR5 or CXCR4 results in the dissociation of gp120 fro gp41 and the formation of a hairpin structure in gp41 that is thrust into the host cell membrane, ultimately allowing for membrane merging, eventual fusion, and entrance of HIV into the target cell (Gallo et al).
Viral tropism determines which host cells will become infected based on which cellular receptors, or co-receptors, they display on their cell membranes. In the case of HIV, tropism determines which cellular co-receptor, CXCR4 or CCR5, the virus will use to help mediate its entry into its host cell. Both CXCR4 and CCR5 are seven-transmembrane-domain G-protein coupled receptors that support membrane fusion by env proteins in a CD4-dependent fashion. CXCR4 serves as the principle entry co-factor for T-cell-line-tropic viruses, or CXCR4-tropic viruses (X4 tropic viruses), while CCR5 serves as the principal entry co-factor for macrophage tropic (M tropic) viruses, or CCR5 tropic viruses (R5 tropic viruses). Naturally following, X4 tropic viruses retain the ability to exclusively infect T cell lines, or circulating activated T cells, R5 tropic viruses retain the ability to exclusively infect macrophages, and X4/R5 tropic viruses have the inclusive ability to infect both T cells lines and macrophages and are thus termed dual tropic (Bjorndal et al).
Clinical Implications of Viral Tropism
Bjorndal et al has experimentally demonstrated the biological phenotype of HIV-1, either rapid replication to high titers of virus (rapid/high) and syncytium inducing (SI), or slow replication to low titers of virus (slow/low) and non-syncytium inducing (NSI), can be determined by the viral tropism, that is whether the viruses are X4 tropic, R5 tropic, or dual tropic (Bjorndal et al). For all HIV-1 subtypes, X4 tropic viruses are perfectly correlated with rapid/high, SI behavior, higher rate of decline of CD4+ T lymphocytes, and early development of AIDS, whereas R5 viruses are associated with slow/low, NSI behavior, much lower rate of decline of CD4+ lymphocytes, asymptomatic infection, and overall delayed onset of AIDS, and dual tropic viruses exhibit the expected intermediary traits. Thus, the biological phenotype of HIV observed in vitro, as well as the tropism of the virus, can be used as a predictive marker of progression of the infection (Bjorndal et al).
Studies of HIV-1 have demonstrated that progression from stages of asymptomatic infection, typically characteristic of R5 tropic viruses, to stages of acquired immunodeficiency, typically characteristic of X4 tropic viruses, is accompanied by the gradual increase in the ability to induce syncytia in peripheral blood mononuclear cells (PBMC) and the ability to replicate in T cell lines, each of which is characteristic of X4 tropic viruses (Bjorndal et al). Essentially, the course of HIV-1 infection often includes a switch in co-receptor usage from the use of R5 to the use of X4 approximately eight to ten years after the initial time of infection as the disease transitions from asymptomatic or mild infection to acquisition of immunodeficiency, a change marking the onset of expanded target cell range and worsened clinical prognosis (Pastore et al). This co-receptor switching is made possible by the generation of genetic diversity with env, specifically in the V3 loop.
The V3 loop, or the third variable region within env involved directly in co-receptor binding, is the region of amino acids whose overall net charge can predict the tropism of the virus. The net charge of the V3 loop is determined by summing the positive and negative amino acid residues within the V3 loop. Typically, a net charge of less than +5 corresponds with an R5 tropic virus, whereas a net charge of equal to or greater than +5 corresponds with an X4 tropic virus. In addition to the overall net charge of the V3 loop, the tropism of the virus is strongly influenced by the charge of the amino acids in two very specific, distinct locations: amino acid positions 11 and 25. The presence of a neutral or uncharged amino acid in position number 11, typically either serine or glycine, is indicative of an R5 tropic virus whereas the presence of a positively charged amino acid in position number 11, typically either arginine or lysine, is indicative of an X4 tropic virus. In position number 25, the presence of a negatively charged amino acid, namely aspartate or glutamate, is indicative of an R5 tropic virus and the presence of an uncharged amino acid in this position is indicative of an X4 tropic virus (Delobel et al). The information regarding the overall net charge of the V3 loop, along with the charges at specific loci within the V3 loop, can thus be used to accurately predict viral tropism.
The delta 32 mutation is a mutation that involves the deletion of 32 base pairs in the CCR5 protein that is present on the surface of macrophages, rendering the protein non-functional. Individuals who are homozygous for this mutation are therefore relatively resistant to infection by CCR5-tropic HIV. Individuals who are heterozygous for this mutation are shown to still be able to get infected with R5 tropic HIV, but their infection and overall viral replication progresses much more slowly than normal HIV positive individuals without this mutation.
Treatment of HIV: Antiretrovirals
Antiretroviral medications have completely revolutionized the prognosis and life expectancy of those living with HIV. Antiretrovirals extremely effective when taken as prescribed. Research scientists are constantly on the lookout for new ways to target the virus, and thus new antiretrovirals are constantly being reviewed for release to the general public. An inclusive list of all of the classes of antiretroviral drugs specific to treating HIV infection, each with a specific example, is as follows:
|Protease inhibitors, which work to inhibit the viral enzyme protease from cleaving gp160 into gp120 and gp41||Ritonavir||Norvir|
|Integrase inhibitors, which work to inhibit the viral enzyme integrase so that viral cDNA can not be integrated into the host genome||Raltegravir||Isentress|
|Nucleoside analogs, which work to inhibit the activity of reverse transcriptase by preventing the enzyme from reverse transcribing viral RNA into proviral DNA||Zidovudine/AZT||Retrovir|
|Entry/fusion inhibitors, which work by blocking HIV from entering cells||Maraviroc||Selzentry|
|Nucleotide analogs, which also work to inhibit the activity of reverse transcriptase by preventing the enzyme from reverse transcribing viral RNA into proviral DNA||Tenofovir||Viread|
|Non-nucleoside inhibitors, which also work to inhibit the activity of reverse transcriptase by preventing the enzyme from reverse transcribing viral RNA into proviral DNA||Nevirapine||Viramune|
Recently, an adenovirus-vector HIV vaccine candidate was tested in clinical trials and was found to actually increase recipient's susceptibility to contracting HIV. No current vaccine is available to prevent against HIV infection of any subtype. However, a vaccine is available to protect cats against Feline Leukemia Virus.