Failure to detect human immunodeficiency virus type 1 superinfection in 28 HIV-seroconcordant individuals with high risk of reexposure to the virus

Multiple Infection and Human Immunodeficiency Virus Superinfection Among Persons who Inject Drugs in Indonesia and Ukraine

BACKGROUND

The HIV Prevention Trials Network (HPTN) 074 study evaluated an integrated human immunodeficiency virus (HIV) treatment and prevention strategy among persons who inject drugs (PWID) in Indonesia, Ukraine, and Vietnam. We previously detected multiple HIV infection in 3 of 7 (43%) of seroconverters with 3-8 HIV strains per person. In this report, we analyzed multiple HIV infection and HIV superinfection (SI) in the HPTN 074 cohort.

METHODS

We analyzed samples from 70 participants in Indonesia and Ukraine who had viral load >400 copies/mL at enrollment and the final study visit (median follow-up, 2.5 years). HIV was characterized with Sanger sequencing, next-generation sequencing, and phylogenetic analysis. Additional methods were used to characterize a rare case of triple-variant SI.

RESULTS

At enrollment, multiple infection was detected in only 3 of 58 (5.2%) participants with env sequence data. SI was detected in only 1 of 70 participants over 172.3 person-years of follow-up (SI incidence, 0.58/100 person-years [95% confidence interval, .015-3.2]). The SI case involved acquisition of 3 HIV strains with rapid selection of a strain with a single pol region cluster.

CONCLUSIONS

These data from a large cohort of PWID suggest that intrahost viral selection and other factors may lead to underestimation of the frequency of multiple HIV infection and SI events.

Harnessing Natural Mosaics: Antibody-Instructed, Multi-Envelope HIV-1 Vaccine Design

The year 2021 marks the 40th anniversary since physicians recognized symptoms of the acquired immunodeficiency syndrome (AIDS), a disease that has since caused more than 30 million deaths worldwide. Despite the passing of four decades, there remains no licensed vaccine for the human immunodeficiency virus type 1 (HIV-1), the etiologic agent of AIDS. Despite the development of outstanding anti-retroviral drugs, there are currently more than one-half million deaths each year due to AIDS. Here, we revisit a conventional vaccine strategy used for protection against variable pathogens like HIV-1, which combines an array of diverse surface antigens. The strategy uses antibody recognition patterns to categorize viruses and their surface antigens into groups. Then a leader is assigned for each group and group leaders are formulated into vaccine cocktails. The group leaders are ‘natural mosaics’, because they share one or more epitope(s) with each of the other group members. We encourage the application of this conventional approach to HIV-1 vaccine design. We suggest that the partnering of an antibody-instructed envelope cocktail with new vaccine vectors will yield a successful vaccine in the HIV-1 field.

How Basic Immunological Principles May Instruct the Design of a Successful HIV-Type 1 Vaccine

This article is dedicated to Dr. Peter Doherty. While Peter continues to make groundbreaking discoveries in the field of immunology, he also provides outstanding scientific mentorship to his trainees. Here we contemplate our past training with Peter, Peter's teachings of basic immunological principles, and how basic principles may instruct the design of a successful human immunodeficiency virus-type 1 vaccine.

Vaccine Design Informed by Virus-Induced Immunity

When an individual is exposed to a viral pathogen for the first time, the adaptive immune system is naive and cannot prevent virus replication. The consequence may be severe disease. At the same time, the host may rapidly generate a pathogen-specific immune response that will prevent disease if the virus is encountered again. Parvovirus B19 provides one such example. Children with sickle cell disease can experience life-threatening transient aplastic crisis when first exposed to parvovirus B19, but an effective immune response confers lifelong protection. We briefly examine the induction and benefits of virus-induced immunity. We focus on three human viruses for which there are no licensed vaccines (respiratory syncytial virus, human immunodeficiency virus type 1, and parvovirus B19) and consider how virus-induced immunity may inform successful vaccine design.

Reducing Risk: Counseling Men Infected with HIV Who Have Sex with Men on Safer Sex Practices with Seroconcordant Partners

The incidence of new HIV infections in the United States continues to be greatest among men who have sex with men (MSM). MSM infected with HIV often seek seroconcordant sexual partners based on intent to limit psychosocial, legal, and health risks they perceive as higher with serodiscordant sexual partners. However, the rationales for limiting sexual relationships exclusively with other MSM infected with HIV may be rooted in misinformation or misperception. Thus, these clients may have a unique sexual health knowledge deficit that nurses, social workers, and other clinicians need to address to help them reduce risk. This article focuses on sexually related health risks that are distinct to MSM infected with HIV seroconcordant partners. Data on the most recent HIV-infection incidence rates in MSM in the United States is provided. Discussion concentrates on the risk these individuals may have in communicating and acquiring sexually transmitted diseases other than HIV, the risk of HIV superinfection, and how sexually transmitted diseases affect persons who are immunocompromised differently than those who are immunocompetent. Finally, recommendations for healthcare professionals who counsel MSM infected with HIV in sexual decision making is provided.

Immunological and virological characteristics of human immunodeficiency virus type 1 superinfection: implications in vaccine design

Superinfection is frequently detected among individuals infected by human immunodeficiency virus type I (HIV-1). Superinfection occurs at similar frequencies at acute and chronic infection stages but less frequently than primary infection. This observation indicates that the immune responses elicited by natural HIV-1 infection may play a role in curb of superinfection; however, these responses are not sufficiently strong to completely prevent superinfection. Thus, a successful HIV-1 vaccine likely needs to induce more potent and broader immune responses than those elicited by primary infection. On the other hand, potent and broad neutralization responses are more often detected after superinfection than during monoinfection. This suggests that broadly neutralizing antibodies are more likely induced by sequential immunization of multiple different immunogens than with only one form of envelope glycoprotein immunogens. Understanding why the protection from superinfection by immunity induced by primary infection is insufficient and if superinfection can lead to cross-reactive immune responses will be highly informative for HIV-1 vaccine design.

Murine Monoclonal Antibodies for Antigenic Discrimination of HIV-1 Envelope Proteins

In the influenza virus field, antibody reagents from research animals have been instrumental in the characterization of antigenically distinct hemagglutinin and neuraminidase membrane molecules. These small animal reagents continue to support the selection of components for inclusion in human influenza virus vaccines. Other cocktail vaccines against variant pathogens (e.g., polio virus, pneumococcus) are similarly designed to represent variant antigens, as defined by antibody reactivity patterns. However, a vaccine cocktail comprising diverse viral membrane antigens defined in this way has not yet been advanced to a clinical efficacy study in the HIV-1 field. In this study, we describe the preparation of mouse antibodies specific for HIV-1 gp140 or gp120 envelope molecules. Our experiments generated renewable reagents able to discriminate HIV-1 envelopes from one another. Monoclonals yielded more precise discriminatory capacity against their respective immunogens than did a small panel of polyclonal human sera derived from recently HIV-1-infected patients. Perhaps these and other antibody reagents will ultimately support high-throughput cartography studies with which antigenically-distinct envelope immunogens may be formulated into a successful HIV-1 envelope cocktail vaccine.

Frequency and implications of HIV superinfection

HIV superinfection occurs when an individual with HIV is infected with a new distinct HIV viral strain. Superinfection has been reported throughout the world, and studies have recorded incidence rates of 0-7·7% per year. Use of next-generation sequencing has improved detection of superinfection, which can be transmitted by injecting drug use and sexual intercourse. Superinfection might have incidence rates comparable to those of initial HIV infection. Clinicians should encourage safe sexual and injecting drug use practices for HIV-infected patients because superinfection has detrimental effects on clinical outcomes and could pose a concern for large-scale antiretroviral treatment plans. The occurrence of superinfection has implications for vaccine research, since it seems initial HIV infection is not fully protective against a subsequent infection. Additional collaborative research could benefit care of patients and inform future vaccine design.

A gp41-based heteroduplex mobility assay provides rapid and accurate assessment of intrasubtype epidemiological linkage in HIV type 1 heterosexual transmission Pairs

A critical step in HIV-1 transmission studies is the rapid and accurate identification of epidemiologically linked transmission pairs. To date, this has been accomplished by comparison of polymerase chain reaction (PCR)-amplified nucleotide sequences from potential transmission pairs, which can be cost-prohibitive for use in resource-limited settings. Here we describe a rapid, cost-effective approach to determine transmission linkage based on the heteroduplex mobility assay (HMA), and validate this approach by comparison to nucleotide sequencing. A total of 102 HIV-1-infected Zambian and Rwandan couples, with known linkage, were analyzed by gp41-HMA. A 400-base pair fragment within the envelope gp41 region of the HIV proviral genome was PCR amplified and HMA was applied to both partners' amplicons separately (autologous) and as a mixture (heterologous). If the diversity between gp41 sequences was low (<5%), a homoduplex was observed upon gel electrophoresis and the transmission was characterized as having occurred between partners (linked). If a new heteroduplex formed, within the heterologous migration, the transmission was determined to be unlinked. Initial blind validation of gp-41 HMA demonstrated 90% concordance between HMA and sequencing with 100% concordance in the case of linked transmissions. Following validation, 25 newly infected partners in Kigali and 12 in Lusaka were evaluated prospectively using both HMA and nucleotide sequences. Concordant results were obtained in all but one case (97.3%). The gp41-HMA technique is a reliable and feasible tool to detect linked transmissions in the field. All identified unlinked results should be confirmed by sequence analyses.

What HIV-positive MSM want from sexual risk reduction interventions: findings from a qualitative study

To facilitate the development of a tailored intervention that meets the needs of HIV-positive men who have sex with men (HIV-positive MSM), we conducted formative research with 52 HIV-positive MSM. We sought to (a) identify major barriers to consistent condom use, (b) characterize their interest in sexual risk reduction interventions, and (c) elicit feedback regarding optimal intervention format. Men identified several key barriers to consistent condom use, including treatment optimism, lessened support for safer sex in the broader gay community, challenges communicating with partners, and concerns about stigmatization following serostatus disclosure. Many men expressed an interest in health promotion programming, but did not want to participate in an intervention focusing exclusively on safer sex. Instead, they preferred a supportive group intervention that addresses other coping challenges as well as sexual risk reduction. Study results reveal important considerations for the development of appealing and efficacious risk reduction interventions for HIV-positive MSM.

Timing and source of subtype-C HIV-1 superinfection in the newly infected partner of Zambian couples with disparate viruses

Background

HIV-1 superinfection occurs at varying frequencies in different at risk populations. Though seroincidence is decreased, in the negative partner of HIV-discordant couples after joint testing and counseling in the Zambia Emory HIV Research Project (ZEHRP) cohort, the annual infection rate remains relatively high at 7-8%. Based on sequencing within the gp41 region of each partner's virus, 24% of new infections between 2004 and 2008 were the result of transmission from a non-spousal partner. Since these seroconvertors and their spouses have disparate epidemiologically-unlinked viruses, there is a risk of superinfection within the marriage. We have, therefore, investigated the incidence and viral origin of superinfection in these couples.

Results

Superinfection was detected by heteroduplex mobility assay (HMA), degenerate base counting of the gp41 sequence, or by phylogenetic analysis of the longitudinal sequences. It was confirmed by full-length env single genome amplification and phylogenetic analysis. In 22 couples (44 individuals), followed for up to five years, three of the newly infected (initially HIV uninfected) partners became superinfected. In each case superinfection occurred during the first 12 months following initial infection of the negative partner, and in each case the superinfecting virus was derived from a non-spousal partner. In addition, one probable case of intra-couple HIV-1 superinfection was observed in a chronically infected partner at the time of his seroconverting spouse's initial viremia. Extensive recombination within the env gene was observed following superinfection.

Conclusions

In this subtype-C discordant couple cohort, superinfection, during the first year after HIV-1 infection of the previously negative partner, occurred at a rate similar to primary infection (13.6% [95% CI 5.2-34.8] vs 7.8% [7.1-8.6]). While limited intra-couple superinfection may in part reflect continued condom usage within couples, this and our lack of detecting newly superinfected individuals after one year of primary infection raise the possibility that immunological resistance to intra-subtype superinfection may develop over time in subtype C infected individuals.

HIV-1 superinfection

PURPOSE OF REVIEW

This review describes the nature and frequency of HIV-1 superinfection and provides advice regarding counselling of patients in accordance with national guidelines.

RECENT FINDINGS

Recent studies have demonstrated conflicting results, from no superinfection to an incidence of over 18%. We discuss the difficulties comparing studies due to population and methodological differences.

SUMMARY

HIV-infected individuals should be counselled that there is risk of superinfection at all stages of HIV, but this is unlikely to be clinically significant unless transmission of resistance occurs. The risk may be as high as the risk of new incident infection in the presence of on-going exposure.

Retroviral coinfections: HIV and HTLV: taking stock of more than a quarter century of research

Retroviral coinfections with HIV-1 and HTLV-1 or with HIV-1 and HTLV-2 occur with variable frequencies throughout the world with the highest prevalence in large metropolitan areas in the Americas, Europe, and Africa. The recognition that retroviral coinfections exist dates back to the discovery of HIV-1 over 25 years ago. Despite the large body of published information regarding the biological and clinical significance of retroviral coinfections, controversy throughout several decades of research was fueled by several flawed epidemiologic studies and anecdotal reports that were not always supported with ample statistical and scientific evidence. However, the growing consensus obtained from recent systematic and well-devised research provides support for at least three conclusions: (1) HIV-1 and HTLV-1 coinfections are often seen in the context of patients with high CD4(+) T cell counts presenting with lymphoma or neurological complications; (2) HIV-1 and HTLV-2 coinfections have been linked in some cases to a "long term nonprogressor" phenotype; and (3) differential function and/or overexpression of the HTLV-1 and HTLV-2 Tax proteins likely play a pivotal role in the clinical and immunologic manifestations of HIV/HTLV-1 and -2 coinfections. This review will recount the chronology of work regarding retroviral coinfections from 1983 through the present.

SHIV infection protects against heterologous pathogenic SHIV challenge in macaques: a gold-standard for HIV-1 vaccine development?

A current debate in the HIV-1 vaccine field concerns the ability of an immunodeficiency virus to elicit a protective response. One argument is that HIV-1 superinfections are frequent in healthy individuals, because virus evades conventional immune surveillance, a serious obstacle to vaccine design. The opposing argument is that protection from superinfection is significant, reflecting a robust immune response that might be harnessed by vaccination to prevent disease. In an experiment designed to address the debate, two macaques received an I.V. inoculation with SHIV KU-1-d (a derivative of SHIV KU-1) and were rested for >10 months. Infection elicited diverse neutralizing antibody activities in both animals. Animals were then exposed to SHIV 89.6P (I.V.), a virus carrying a heterologous envelope protein relative to the vaccine strain. Infection was monitored by viral load and CD4+ T-cell measurements. All control animals were infected and most succumbed to disease. In contrast, protection from superinfection was statistically significant in test monkeys; one animal showed no evidence of superinfection at any time point and the second showed evidence of virus at only one time point over a 6-month observation period. Neither animal showed signs of disease. Perhaps this protective state may serve as a 'gold-standard' for HIV-1 vaccine development, as a similar degree of protection against immunodeficiency virus infections in humans would be much desired.

HIV-1 vaccine design: harnessing diverse lymphocytes to conquer a diverse pathogen

In the fall of 2007, the HIV-1 research field received news that their front-runner vaccine was not protective. In response to this disappointment, scientists are now reviewing the intricacies of the immune response toward HIV-1 to develop new and better strategies for vaccine development. Decades ago, researchers recognized the impressive amino acid and carbohydrate diversity of HIV-1, and the associated obstacles to vaccine development. At first glance, the diversity and other unique features of HIV-1 may seem insurmountable, but attention to vaccine successes in other fields serves to renew optimism. The newly-licensed rotavirus and papillomavirus cocktail vaccines remind scientists that diverse pathogens can be conquered and that the chronic nature of a virus infection need not thwart successful vaccine design. Here we describe current efforts to gain insights from other vaccine fields and to adopt a cocktail vaccine approach for the prevention of HIV-1 infections in humans.

Preclinical and clinical development of a multi-envelope, DNA-virus-protein (D-V-P) HIV-1 vaccine

The human immune system has evolved to recognize antigenic diversity, a strength that has been harnessed by vaccine developers to combat numerous pathogens (e.g., pneumococcus, influenza virus, rotavirus). In each case, vaccine cocktails were formulated to include antigenic variants of the target. To combat HIV-1 diversity, we assembled a cocktail vaccine comprising dozens of envelopes, delivered as recombinant DNA, vaccinia virus, and protein for testing in a clinical trial. One vaccinee has now completed vaccinations with no serious adverse events. Preliminary analyses demonstrate early proof-of-principle that a multi-envelope vaccine can elicit neutralizing antibody responses toward heterologous HIV-1 in humans.

Partial protection of Simian immunodeficiency virus (SIV)-infected rhesus monkeys against superinfection with a heterologous SIV isolate

Although there is increasing evidence that individuals already infected with human immunodeficiency virus type 1 (HIV-1) can be infected with a heterologous strain of the virus, the extent of protection against superinfection conferred by the first infection and the biologic consequences of superinfection are not well understood. We explored these questions in the simian immunodeficiency virus (SIV)/rhesus monkey model of HIV-1/AIDS. We infected cohorts of rhesus monkeys with either SIVmac251 or SIVsmE660 and then exposed animals to the reciprocal virus through intrarectal inoculations. Employing a quantitative real-time PCR assay, we determined the replication kinetics of the two strains of virus for 20 weeks. We found that primary infection with a replication-competent virus did not protect against acquisition of infection by a heterologous virus but did confer relative control of the superinfecting virus. In animals that became superinfected, there was a reduction in peak replication and rapid control of the second virus. The relative susceptibility to superinfection was not correlated with CD4(+) T-cell count, CD4(+) memory T-cell subsets, cytokine production by virus-specific CD8(+) or CD4(+) cells, or neutralizing antibodies at the time of exposure to the second virus. Although there were transient increases in viral loads of the primary virus and a modest decline in CD4(+) T-cell counts after superinfection, there was no evidence of disease acceleration. These findings indicate that an immunodeficiency virus infection confers partial protection against a second immunodeficiency virus infection, but this protection may be mediated by mechanisms other than classical adaptive immune responses.

Molecular and contextual markers of hepatitis C virus and drug abuse

The spread of hepatitis C virus (HCV) infection involves a complex interplay of social risks, and molecular factors of both virus and host. Injection drug abuse is the most powerful risk factor for HCV infection, followed by sexual transmission and additional non-injection drug abuse factors such as co-infection with other viruses and barriers to treatment. It is clearly important to understand the wider context in which the factors related to HCV infection occur. This understanding is required for a comprehensive approach leading to the successful prevention, diagnosis, and treatment of HCV. An additional consideration is that current treatments and advanced molecular methods are generally unavailable to socially disadvantaged patients. Thus, the recognition of behavioral/social, viral, and host factors as components of an integrated approach to HCV is important to help this vulnerable group. Equally important, this approach is key to the development of personalized patient treatment - a significant goal in global healthcare. In this review, we discuss recent findings concerning the impact of drug abuse, epidemiology, social behavior, virology, immunopathology, and genetics on HCV infection and the course of disease.

Human immunodeficiency virus type 1 superinfection occurs despite relatively robust neutralizing antibody responses

Superinfection by a second human immunodeficiency virus type 1 (HIV-1) strain indicates that gaps in protective immunity occur during natural infection. To define the role of HIV-1-specific neutralizing antibodies (NAbs) in this setting, we examined NAb responses in 6 women who became superinfected between approximately 1 to 5 years following initial infection compared to 18 women with similar risk factors who did not. Although superinfected individuals had less NAb breadth than matched controls at approximately 1 year postinfection, no significant differences in the breadth or potency of NAb responses were observed just prior to the second infection. In fact, four of the six subjects had relatively broad and potent NAb responses prior to infection by the second strain. To more specifically examine the specificity of the NAbs against the superinfecting virus, these variants were cloned from five of the six individuals. The superinfecting variants did not appear to be inherently neutralization resistant, as measured against a pool of plasma from unrelated HIV-infected individuals. Moreover, the superinfected individuals were able to mount autologous NAb responses to these variants following reinfection. In addition, most superinfected individuals had NAbs that could neutralize their second viral strains prior to their reinfection, suggesting that the level of NAbs elicited during natural infection was not sufficient to block infection. These data indicate that preventing infection by vaccination will likely require broader and more potent NAb responses than those found in HIV-1-infected individuals.

A sudden rise in viral load is infrequently associated with HIV-1 superinfection

OBJECTIVE

To investigate the association between an unexpected increase in the blood plasma HIV-1 viral load in chronically untreated HIV-infected patients and the occurrence of an HIV superinfection, we analyzed the HIV-1 quasispecies in plasma samples before and at peak level in 14 patients.

RESULTS

Phylogenetic analysis of HIV-1 env-V3 fragments showed that in 2 patients a superinfection had occurred: their dominant V3 population at the peak level clustered separately from the V3 sequences in a sample predating the peak level. The rapid rise in viral load could be attributed to upper respiratory tract infections or a vaccination in 4 patients, suggesting that even minor health problems can result in significantly increased HIV-1 replication. In most other patients, no minor or major medical condition accompanied the rise in HIV-1 viral load, implying that in these patients the viral load increase was probably associated with disease progression.

CONCLUSION

This study suggests that an unexpected rapid rise in the plasma HIV-1 viral load of untreated patients can infrequently be ascribed to an HIV-1 superinfection.

Identifying HIV-1 dual infections

Transmission of human immunodeficiency virus (HIV) is no exception to the phenomenon that a second, productive infection with another strain of the same virus is feasible. Experiments with RNA viruses have suggested that both coinfections (simultaneous infection with two strains of a virus) and superinfections (second infection after a specific immune response to the first infecting strain has developed) can result in increased fitness of the viral population. Concerns about dual infections with HIV are increasing. First, the frequent detection of superinfections seems to indicate that it will be difficult to develop a prophylactic vaccine. Second, HIV-1 superinfections have been associated with accelerated disease progression, although this is not true for all persons. In fact, superinfections have even been detected in persons controlling their HIV infections without antiretroviral therapy. Third, dual infections can give rise to recombinant viruses, which are increasingly found in the HIV-1 epidemic. Recombinants could have increased fitness over the parental strains, as in vitro models suggest, and could exhibit increased pathogenicity. Multiple drug resistant (MDR) strains could recombine to produce a pan-resistant, transmittable virus.

We will describe in this review what is presently known about super- and re-infection among ambient viral infections, as well as the first cases of HIV-1 superinfection, including HIV-1 triple infections. The clinical implications, the impact of the immune system, and the effect of anti-retroviral therapy will be covered, as will as the timing of HIV superinfection. The methods used to detect HIV-1 dual infections will be discussed in detail. To increase the likelihood of detecting a dual HIV-1 infection, pre-selection of patients can be done by serotyping, heteroduplex mobility assays (HMA), counting the degenerate base codes in the HIV-1 genotyping sequence, or surveying unexpected increases in the viral load during follow-up. The actual demonstration of dual infections involves a great deal of additional research to completely characterize the patient's viral quasispecies. The identification of a source partner would of course confirm the authenticity of the second infection.

Routine HIV-1 genotyping as a tool to identify dual infections

OBJECTIVES

The incidence of HIV-1 dual infections is generally thought to be low, but as dual infections have been associated with accelerated disease progression, its recognition is clinically important. Methods to identify HIV-1 dual infections are time consuming and are not routinely performed.

DESIGN

Genotyping of the HIV-1 protease and reverse transcriptase (prot/RT) genes is commonly performed in the western world to detect drug-resistance mutations in clinical isolates. In our hospital, prot/RT baseline sequencing is part of the patient care for all newly infected patients in the Amsterdam region since 2003. We reasoned that degenerate base codes in this sequence could indicate either extensive viral evolution or infection with multiple HIV-1 strains.

METHODS

We amplified, cloned and sequenced multiple HIV-1 envelope (env)-V3 and gag sequences from patients with 34 or more (range 34-99) degenerate base codes in the ViroSeq genotyping RT sequence (37 out of 1661 available records) to estimate the number of HIV-1 dual infections in this group.

RESULTS

Of the 37 patients included in this study, 16 (43.2%, equal to 1% of the 1661 total records) had an HIV-1 dual infection based on phylogenetic analysis of env-V3/gag sequences. If only sequences with 45 or more degenerate base codes were taken into account, 73.3% of patients showed evidence of a dual infection.

CONCLUSION

We describe an additional use of routinely performed HIV-1 genotyping. In patients with a high number of degenerate bases (> or = 34) in RT it is important to consider the possibility of a dual HIV-1 infection.

Why do B cells mutate their immunoglobulin receptors?

B cells have the unique ability to acquire large numbers of point mutations in the variable segment of rearranged immunoglobulin (Ig) genes during a germinal center reaction. It is broadly accepted that somatic hypermutation (SHM) and affinity maturation are required to generate memory B cells and to produce antibodies capable of accomplishing the host defense functions of the humoral component of the adaptive immune system. However, several studies illustrate that low-avidity interactions between antigen and the B-cell receptor can induce deletion, receptor editing and a T-dependent immune response, suggesting that the high-avidity binding of antigen is not essential. If enhanced antigen binding is not essential for immune responses, what is the purpose of SHM? An alternative benefit of SHM might be to enhance the ability of B cells to track antigens expressed by rapidly mutating microorganisms.

Retroviral superinfection resistance

The retroviral phenomenon of superinfection resistance (SIR) defines an interference mechanism that is established after primary infection, preventing the infected cell from being superinfected by a similar type of virus. This review describes our present understanding of the underlying mechanisms of SIR established by three characteristic retroviruses: Murine Leukaemia Virus (MuLV), Foamy Virus (FV), and Human Immunodeficiency Virus (HIV). In addition, SIR is discussed with respect to HIV superinfection of humans. MuLV resistant mice exhibit two genetic resistance traits related to SIR. The cellular Fv4 gene expresses an Env related protein that establishes resistance against MuLV infection. Another mouse gene (Fv1) mediates MuLV resistance by expression of a sequence that is distantly related to Gag and that blocks the viral infection after the reverse transcription step. FVs induce two distinct mechanisms of superinfection resistance. First, expression of the Env protein results in SIR, probably by occupancy of the cellular receptors for FV entry. Second, an increase in the concentration of the viral Bet (Between-env-and-LTR-1-and-2) protein reduces proviral FV gene expression by inhibition of the transcriptional activator protein Tas (Transactivator of spumaviruses). In contrast to SIR in FV and MuLV infection, the underlying mechanism of SIR in HIV-infected cells is poorly understood. CD4 receptor down-modulation, a major characteristic of HIV-infected cells, has been proposed to be the main mechanism of SIR against HIV, but data have been contradictory. Several recent studies report the occurrence of HIV superinfection in humans; an event associated with the generation of recombinant HIV strains and possibly with increased disease progression. The role of SIR in protecting patients from HIV superinfection has not been studied so far. The phenomenon of SIR may also be important in the protection of primates that are vaccinated with live attenuated simian immunodeficiency virus (SIV) against pathogenic SIV variants. As primate models of SIV infection closely resemble HIV infection, a better knowledge of SIR-induced mechanisms could contribute to the development of an HIV vaccine or other antiviral strategies.

Evidence for frequent reinfection with human immunodeficiency virus type 1 of a different subtype

A major premise underlying current human immunodeficiency virus type 1 (HIV-1) vaccine approaches is that preexisting HIV-1-specific immunity will block or reduce infection. However, the recent identification of several cases of HIV-1 reinfection suggests that the specific immune response generated for chronic HIV-1 infection may not be adequate to protect against infection by a second HIV-1 strain. It has been unclear, though, whether these individuals are representative of the global epidemic or are rare cases. Here we show that in a population of high-risk women, HIV-1 reinfection occurs almost as commonly as first infections. The study was designed to detect cases of reinfection by HIV-1 of a different subtype and thus captured cases where there was considerable diversity between the first and second strain. In each case, the second virus emerged approximately 1 year after the first infection, and in two cases, it emerged when viral levels were high, suggesting that a well-established HIV-1 infection may provide little benefit in terms of immunizing against reinfection, at least by more-divergent HIV-1 variants. Our findings indicate an urgent need for studies of larger cohorts to determine the incidence and timing of both intersubtype and intrasubtype reinfection.