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Brazilian Journal of Infectious Diseases

Print version ISSN 1413-8670On-line version ISSN 1678-4391

Braz J Infect Dis vol.7 no.1 Salvador Feb. 2003

http://dx.doi.org/10.1590/S1413-86702003000100002 

MINI-REVIEW

 

Diagnosis, antiretroviral therapy, and emergence of resistance to antiretroviral agents in HIV-2 infection: a review

 

 

Maia Hightower; Esper Georges Kallas

Infectious Diseases Discipline, Federal University of São Paulo, São Paulo, SP, Brazil

Correspondence

 

 


ABSTRACT

Human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) are the causative agents of AIDS. HIV-2 is prevalent at moderate to high rates in West African countries, such as Senegal, Guinea, Gambia, and Cape Verde. Diagnosis of HIV-2 is made with a positive HIV-1/HIV-2 ELISA or simple/rapid assay, followed by one or two confirmatory tests specific for HIV-2. Following CD4+ T cell counts, HIV-2 viral burden and clinical signs and symptoms of immunodeficiency are beneficial in monitoring HIV-2 disease progression. Although non-nucleoside reverse transcriptase inhibitors are ineffective in treating HIV-2, nucleoside reverse transcriptase inhibitors and protease inhibitors can be effective in dual and triple antiretroviral regimens. Their use can decrease HIV-2 viral load, increase CD4+ T cell counts and improve AIDS-related symptoms. HIV-2 resistance to various nucleoside reverse transcriptase inhibitors and protease inhibitors, including zidovudine, lamivudine, ritonavir and indinavir, has been identified in some HIV-2 infected patients on antiretroviral therapy. The knowledge of HIV-2 peculiarities, when compared to HIV-1, is crucial to helping diagnose and guide the clinician in the choice of the initial antiretroviral regimen and for monitoring therapy success.

Key Words: HIV-2, antiretroviral treatment, review.


 

 

Human Immunodeficiency Virus Type 2 (HIV-2) and Human Immunodeficiency Virus Type 1 (HIV-1) are the causative agents of AIDS. HIV-2 was first identified in 1986 from a West African patient with AIDS, two years after the identification of HIV-1. HIV-2 is prevalent at moderate to high rates in West African countries, such as Senegal, Guinea, Gambia, and Cape Verde and is the leading cause of AIDS in Guinea-Bissau [1]. It is relatively rare outside of West Africa [2], although individual cases have been described in other parts of Africa, Europe, the Americas, and Asia, with most cases of HIV-2 infection having some epidemiological link to West Africa [3]. Recently, HIV-2 transmission in Brazil was documented in a Brazilian woman who had been sexually exposed to a man from Guinea-Bissau [4]. It is now known that at least seven phylogenetic subtypes of HIV-2 exist, including HIV-2 subtype A, predominant in Guinea-Bissau, and HIV-2 subtype B, predominant in the Ivory Coast [5-9].

HIV-2 is transmitted the same way as HIV-1, mainly through sexual contact, IV drug use, perinatally, and by contact with contaminated blood products[2]. HIV-2 appears to be less virulent than HIV-1, with a longer latent period before progression to AIDS[10], lower plasma viral loads, slower decline in CD4+ T cell count, lower mortality, lower heterosexual transmission, is rarely transmitted vertically, and is possibly protective against infection by HIV-1 [11-19]. The data regarding HIV-2 as a protective factor against infection by HIV-1 is being revisited as subsequent studies have shown conflicting results [20, 21]. A recent meta-analysis concluded that HIV-2 is a risk factor, not a protective factor, for HIV-1 infection [22].

A genetic comparison of HIV-1 and HIV-2 reveals a significant difference in amino acid sequences. For instance, the two viruses share only about 60% of the predicted amino acid sequence for the entire pol gene. Despite these genetic differences, the HIV-1 and HIV-2 reverse transcriptase (RT) proteins are similar in overall structure and functionality [23]. This finding has led to the suggestion that at least some of the drugs found to be effective against HIV-1 could also be effective against HIV-2 [24]. Drug classes effective against HIV-1 that may be effective against HIV-2 include protease inhibitors (PIs), nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs). In this review, the diagnosis and monitoring of HIV-2 infection are briefly discussed, followed by an in depth evaluation of the mechanism of action, in vitro studies and case reports for each drug class that may be useful for the treatment of HIV-2 infection.

 

Diagnosis of HIV-2 Infection

Most diagnostic and therapeutic strategies available to monitor and treat HIV were developed specifically for clade B of HIV-1. This poses unique therapeutic challenges for the diagnosis, monitoring, and therapy of patients infected with genetically diverse HIV viruses, including HIV-2. In 1991, the United States Food and Drug Administration licensed the first combination HIV-1/HIV-2 ELISA screening test[25]. Currently available HIV-1/HIV-2 combination tests incorporate antigens from both viruses. The World Health Organization guidelines for the diagnosis of an HIV infection recommend various strategies based on the presence of clinical signs or symptoms and prevalence of HIV infection. A positive HIV-1/HIV-2 ELISA or simple/rapid assay is followed by one or two confirmatory tests specific for either HIV-1 or HIV-2 [26]. Western blot is normally used as a confirmatory test, although combinations of simple/rapid and ELISA tests can be used with reported >99% specificity and sensitivity [27]. Guidelines for HIV-2 Western blot vary by organization. The US Center for Disease Control recommends that each Western blot test be interpreted by the criteria suggested by the kit manufacturer [25]. The World Health Organization guidelines require reactivity to at least two HIV-2 envelope antigens to be considered a positive result [28]. Other organizations require reactivity to a combination of HIV-2 gag (p26, gp34) and env (gp105) antigens.

 

Monitoring HIV-2 Infection

Monitoring patients with HIV-2 is more difficult than monitoring those with HIV-1. A small-scale retrospective analysis of an HIV-2 cohort showed that a proportional increase in HIV-2 viral load burden results in an increased rate of CD4 T cell decline [29]. When equal plasma viral loads are compared between individuals infected with HIV-1 and HIV-2, both groups are found to have a similar rate of CD4+ T cell decline [30] . This data suggests that similar to the HIV-1 model, serial CD4+ T cell count and HIV-2 viral burden are useful for monitoring HIV-2 disease progression. Although there are a number of commercially available assays for the quantification of HIV-1 RNA in plasma, none are able to detect HIV-2 or group O HIV-1 [31]. In the absence of viral load monitoring, the U.S. Centers for Disease Control suggest that physicians use CD4+ T cell counts and other indicators of immune suppression to monitor disease progression and response to treatment.

 

Therapy

Although HIV-1 and HIV-2 are closely related when compared to other retroviruses, the differences that have been detected in the reverse transcriptase and protease genes result in diverse susceptibility to antiretroviral agents, particularly the non-nucleoside reverse transcriptase inhibitors. We have made a detailed review of the mechanisms of action of each class of antiretroviral agent and the mechanism of resistance, focussing on the HIV-2 strains.

 

Nucleoside Reverse Transcriptase Inhibitors

HIV-1 and HIV-2 reverse transcriptase (RT) enzyme regulates the formation of viral DNA prior to integration into the host cell genome. This RT catalyzes the incorporation of deoxynucleoside triphosphate (dNTPs), forming a negative-sense DNA strand, by using HIV-positive sense RNA as a template. The RT RNase activity catalyzes the degradation of the positive-sense RNA from the negative sense DNA. This enzyme's DNA polymerase activity generates a second positive-sense DNA strand to form double-stranded proviral DNA [32]. Nucleoside reverse transcriptase inhibitors (NRTIs) structurally resemble endogenous dNTPs, inhibiting the formation of viral DNA through two mechanisms. First, they competitively inhibit dNTPs for the RT enzyme. Second, once incorporated into the HIV DNA strand, their modified 3' hydroxyl group causes chain termination of DNA synthesis.

In vitro studies of antiretroviral activity against HIV-2 show NRTIs to have inhibitory effects against wild type and mutant HIV-2 clones [33]. The effective concentrations of zidovudine (AZT), lamivudine (3TC), didanosine (ddI), zalcitabine (ddC) and stavudine (d4T) needed to inhibit wild type HIV-2 are very similar to what is effective against wild type HIV-1 [34-38]. In vitro data and the relative safety profile of NRTIs have made them attractive first line agents against HIV-2 infection. Based on a limited number of case reports, antiretroviral therapy against HIV-2 with dual NRTI and triple NRTI/PI-based therapy appears to decrease HIV-2 viral load, increase CD4+ T cell counts and improve AIDS-related symptoms [39-42].

 

Nucleoside Reverse Transcriptase Inhibitor Resistance

HIV-1 NRTI resistance results from base changes within the RT genome, provoking amino acid substitutions in the transcribed enzyme, which in turn confer structural changes at the enzyme active site or at associated functional areas. Each NRTI induces a predictable set of genetic alterations in a step-wise fashion. Primary mutations generally arise first, with secondary mutations developing during continued therapy [32]. Data from modified HIV-2 mutants show that HIV-2 RT resistance can be conferred by mutations at sites homologous to those for HIV-1 RT NRTI resistance. In a cell-free system, site-directed mutagenesis of HIV-2 RT amino acid residues homologous to residues in HIV-1 RT reduces the sensitivity of mutated HIV-2 to NRTIs [33]. The HIV-1 RT mutation T215Y confers resistance to zidovudine. Based on further analysis of HIV-2 mutants, it appears that the homologue S215Y mutant in HIV-2 RT confers similar resistance to zidovudine by repositioning the template-primer [43].

Contrary to data from genetically modified HIV-2 clones, clinical studies of HIV-2 antiviral resistance suggest that the pattern of HIV-2 RT mutations after exposure to NRTIs is less well defined than for HIV-1. Van der Ende et al. conducted a genotypic and phenotypic analysis of HIV-2 clones obtained from individuals before and after initiation of NRTI therapy. [41,44] HIV-2 clones obtained from two individuals who were treated with zidovudine therapy alone were 10 to 20 times less sensitive to inhibition by zidovudine then HIV-2 clones from individuals who were naive to zidovudine. One of the clones had a methionine at position 151 [44]. In HIV-1, the Q151M mutation is associated with multiple drug resistance and generally develops in individuals after more than a year of combined therapy with zidovudine and zalcitabine (ddC) or ddI.[45] No other mutations associated with resistance to zidovudine in HIV-1 infection were found. HIV-2 clones obtained from two HIV-2 infected patients after dual therapy with NRTIs (AZT + ddI; AZT + 3TC) were 10 to 45 times less sensitive to 3TC therapy. Genetic analysis of both clones showed a M184V mutation [44]. In HIV-1, this mutation is associated with strongly reduced sensitivity to 3TC in phenotypic assays [46]. Based on this latter study, it appears that some mutations associated with HIV-1 RT resistance also play a role in HIV-2 resistance. However, the genetic basis of HIV-2 NRTI resistance may not be limited to or include every region homologous to HIV-1 NRTI resistance [44].

Rodes et al. conducted genotypic analysis of 12 HIV-2 infected individuals who had been exposed to antiretroviral drugs for longer than six months [40]. Four individuals were found to carry virus genotypes with amino acid substitutions associated with NRTI resistance in HIV-1. Two patients carried the K70R mutation, which is associated with zidovudine resistance. Two others carried the M184V mutation. Notably, these two individuals also carried the Q151M mutation. Both of these individuals had experienced a decline in CD4+ lymphocyte counts and high viral load values, based on measurements made with two different techniques. The first patient had been treated with ZDV and 3TC for 26 months while the other had been treated with AZT, 3TC, and indinavir (IDV) for 12 months, followed by nevirapine (NVP), saquinavir (SQV), and d4T therapy for one month. This clinical picture supports the idea that they were experiencing NRTI-based treatment failure [40]. Though these two studies have limited data the studies of Smith et al. support these findings; in their study of seven patients with HIV-2, two patients did not respond to antiviral therapy [42]. One patient failed to respond to dual NRTI therapy (AZT and ddI) and the other did not respond to dual ddC/SQV therapy, followed by sequential single-drug switches. Phenotypic resistance studies performed on viruses isolated from this patient after the start of antiretroviral therapy suggested high 3TC and SQV resistance [42].

The clinical findings from these three studies support the conclusion that the mechanism of HIV-2 RT resistance to NRTIs is similar to that of HIV-1. Amino acid changes associated with HIV-1 NRTI resistance at M184 and Q151M also seem to occur in HIV-2, and have been associated with NRTI treatment failure (Table 1). The reasons for the presence of these mutations remain unclear. The significance of other mutations in the HIV-2 RT gene after exposure to NRTI is also unknown. It is also not known if mutations in the NRTI gene accumulate in a step-wise fashion, conferring increasing resistance to NRTI therapy. Further genetic and phenotypic analysis of NRTI drug resistance in HIV-2 is needed.

 

 

Non-nucleoside Reverse Transcriptase Inhibitors (NNRTIs)

The NNRTIs are a structurally diverse group of compounds, with an aromatic structure, that bind to a hydrophobic pocket near the polymerase site of HIV-1 RT. In vitro studies that measure HIV-2 susceptibility to inhibition by NNRTIs have supported the conclusion that these types of drugs have no effect against HIV-2 [47]. This may be due to differences in the amino acid sequence around the two RT NNRTI binding pockets, conferring resistance to NNRTI drugs [47]. Natural resistance of HIV-2 RT to NNRTIs is thought to be conferred by a single amino acid, Leu-188. A single amino acid change at this site renders HIV-2 RT sensitive to some NNRTIs, including efavirenz and delavirdine [48]. Recent in vitro studies have found that the newer NNRTIs, including delavirdine, inhibit HIV-2 at effective concentrations at least 50 fold higher than those that inhibit HIV-1 [24,49]. The majority of these more powerful NNRTIs do not inhibit HIV-2 replication at sub toxic concentrations. Although NNRTIs are considered very safe, with few cytotoxic effects at concentrations effective against HIV-1, the higher concentrations needed to suppress HIV-2 make NNRTIs a questionable therapeutic choice for HIV-2 infected patients.

 

Protease Inhibitors (PI)

HIV-1, HIV-2 and SIV proteases belong to the aspartyl protease family. They are responsible for post-translational processing and cleavage of the polyprotein products PrGAG and PrGAG-POL into functional core proteins and essential enzymes, including reverse transcriptase, integrase, and protease. Proteases are required to produce a mature retrovirus. Protease inhibitors competitively bind to the protease substrate site, resulting in the production of immature, non-infectious particles [50]. Kinetic studies of protease inhibitor binding activity against wild type HIV-2 protease show PIs to be effective against HIV-2, yet they bind with a 10 to 100 times weaker affinity to HIV-2 protease, depending on the inhibitor [51, 52]. Studies of individual PIs show nelfinavir and saquinavir to exert the same inhibitory activity against HIV-2 as against HIV-1, whereas ritonavir and indinavir are one to two orders of magnitude less inhibitory against HIV-2 [53-56]. A limited number of case reports have confirmed that antiretroviral therapy against HIV-2 with dual PI or triple NRTI/PI based therapy results in decreased HIV-2 viral load, increased CD4+ T cell count and an improvement in AIDS related symptoms [42]. There are no published studies that indicate whether the increased concentrations of ritonavir and indinavir needed to suppress HIV-2 in cell culture are clinically significant. Differences in the protease ligand binding mechanism may be the reason for the difference in inhibitory activity between these two retroviruses.

 

Mechanism of Protease Ligand Binding

HIV-1 and HIV-2 proteases have a 45% similarity in their primary protein structure. However, the protease active site residues differ at only three amino acids: 32 (Val—Ile), 47 (Ille—Val), and 82 (Val—Ile). Comparative studies of HIV-1 and HIV-2 protease crystallography structures complexed with identical synthetic protease inhibitors indicate minimal differences in the secondary and tertiary structure of these two proteases [57,58]. The largest structural differences between the HIV-1 and HIV-2 proteases are located at residues 15-20, 34-40, and 65-73, away from the substrate binding sites [58]. It has been suggested that since these regions are not near the binding site region it is unlikely that they have any direct effect on inhibitory activity [58]. However, other studies have suggested that residues outside the active site cavity can confer differences in HIV-1 and HIV-2 structural and kinetic properties [59].

Studies on HIV-1, HIV-2 and SIV protease crystal structures bound to a tripeptide analogue protease inhibitor SB203386 have provided additional information on differences in retroviral protease ligand specificity and structure. SB203386 is a potent competitive inhibitor of HIV-1 (Ki=18nM), however it shows a 2nd order magnitude decreased affinity for HIV-2 protease (Ki=1280nM) and SIV (Ki=960nM) [59]. Comparative analysis of the crystal structures of HIV-1 and HIV-2 proteases complexed with SB203386 reveals different modes of inhibitory binding. In HIV-1 protease, a single inhibitor molecule is found bound to the protease active site. The HIV-2 protease binds two inhibitor molecules, each occupying half of the active site and they bind with decreased affinity. It has been suggested that this difference in the inhibitory binding mechanism is due to differences in the active site residues of HIV-1 and HIV-2/SIV proteases. However, mutating these three residues in HIV-1 protease to their HIV-2 and SIV counterparts does not reduce the PI binding affinity to that of HIV-2 and SIV, nor does it invoke the SIV protease-like inhibitor-binding mode [59,60]. This finding has lead to the hypothesis that inhibitor specificity and mode of binding are partially conferred by residues outside the active site cavity [60]. Studies of an HIV-1 strain with HIV-2 amino acid substitutions at positions 31-37, an HIV-1 and -2 chimera protease named HIV-1 (2:31-37), suggest that decreased affinity to SB203386 is conferred by the amino acids in the 31-37 position of the HIV-2 protease region ("the 30's loop") [61]. Crystallographic analysis of the HIV-1 (2:31-37) chimera reveals that the 30's loop renders the chimera similar to HIV-2 protease, both in terms of inhibitory binding affinity and the two inhibitory molecules per protease in the dimer mode of binding [62]. Differences in ligand binding between HIV-1 and HIV-2 protease may explain kinetic differences between these two proteases. What remains unclear is if differences in the mechanism of SB203386 ligand binding are the sole explanation for differences in protease inhibitory kinetic and in vitro studies.

 

Protease Inhibitor Resistance

PI resistance to HIV-1 is not completely understood. Mutations that confer drug resistance have been identified in protease genes. Mutations of the HIV-1 protease gene that confer resistance to specific protease inhibitors include L90M induced by saquinavir, M46I and V82A/P induced by ritonavir and indinavir, and D30N induced by nelfinavir [63]. As mentioned previously, the HIV-2 protease active site differs at 82 (Val—Ile), which may confer natural HIV-2 resistance to ritonavir and indinavir. It has been suggested that the presence of the D30N mutation as a natural polymorphism in HV-2 reduces the efficacy of nelfinavir against HIV-2 [42]. A genotypic analysis of four HIV-2 individuals exposed to PI therapy demonstrated the M46I mutation in all four individuals. Minor resistance mutations, as well as new mutations not previously reported to be associated with PI resistance in HIV-1, were also found. One patient was also found to harbor the V82F mutation. This patient showed a progressive decline in CD4+ lymphocytes, despite being treated with indinavir (Table 1) [40]. In a case report of two patients infected with both HIV-1 and HIV-2, antiretroviral therapy successfully suppressed HIV-1, but failed to suppress HIV-2 [64]. One of the patients was being treated with ritonavir and saquinavir, while the other was treated with AZT, lamivudine and nelfinavir. No genotypic analysis results were available. The first patient never experienced a decline in HIV-2 viral load. The second patient experienced an initial decline in HIV-2 viral load, followed by a rebound to pre-antiretroviral therapy levels. The available comparative studies of HIV-1 and HIV-2 antiretroviral effectiveness have not evaluated coinfections with HIV-1 and HIV-2. Based on what is know about these two viruses, we predict that the first patient would have suppressed HIV-2, though perhaps less than for HIV-1. The lack of HIV-2 suppression when there is adequate HIV-1 response to therapy suggests preferential HIV-1 ligand binding. This possibility is supported by the findings for the second patient. The initial HIV-2 viral suppression followed by a rebound suggests the development of multi-NRTI resistance when there is an inadequate PI response, or the acquisition of PI resistance mutation(s) during the course of therapy.

 

Conclusions

In conclusion, many questions regarding the epidemiology, diagnosis, monitoring and treatment of HIV-2 remain unanswered. Epidemiological data must always be considered to provide insights for an early diagnosis of HIV-2 infection. The currently available data supports initiation of HIV-2 therapy with an NRTI and PI based triple antiretroviral regimen, considering the described natural resistance to NNRTI. As observed in HIV-1 infection, dual NRTI therapy alone may be associated with the development of NRTI resistance by HIV-2. Cohort studies of patients infected with HIV-2 have been extremely limited and have not addressed central questions regarding therapy for this virus, mostly due to the small number of publications and case series. No large clinical trials investigating the most effective types of antiretroviral therapies for HIV-2 are available. Therefore, clinicians treating HIV-2-infected patients should use the experience accumulated when treating HIV-1, and available HIV-2 in vitro susceptibility data, as well as case series, to guide therapeutic decisions. As developing counties in Africa increase the utilization of antiretroviral therapy as part of their overall HIV/AIDS prevention and treatment program and HIV-2 infection is more frequently found outside of Africa, clinical studies that test for effective treatment options are needed.

 

Acknowledgements

M. Hightower was supported by The Rotary International Ambassadorial Scholars Program.

 

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Correspondence to
Dr. Esper Georges Kallas
M.D., Ph.D. Laboratório de Imunologia, Disciplina de Doenças Infecciosas e Parasitárias, Escola Paulista de Medicina/UNIFESP
Rua Pedro de Toledo 781, 15 andar
04039-032 - São Paulo - SP, Brazil
E-mail: kallas.dmed@epm.br

Received on 13 May 2002
Revised 20 December 2002

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