Mother-to-infant transmission of simian immunodeficiency virus is rare in sooty mangabeys and is associated with low viremia

Milk Transmission of Mammalian Retroviruses

The transmission of viruses from one host to another typically occurs through horizontal or vertical pathways. The horizontal pathways include transmission amongst individuals, usually through bodily fluids or excretions, while vertical transmission transpires from mother to their offspring, either during pregnancy, childbirth, or breastfeeding. While there are more than 200 human pathogenic viruses to date, only a small number of them are known to be transmitted via breast milk, including cytomegalovirus (CMV), human immunodeficiency virus type 1 (HIV-1), and human T cell lymphotropic virus type 1 (HTLV-1), the latter two belonging to the family Retroviridae. Breast milk transmission is a common characteristic among mammalian retroviruses, but there is a lack of reports summarizing our knowledge regarding this route of transmission of mammalian retroviruses. Here, we provide an overview of the transmission of mammalian exogenous retroviruses with a focus on Orthoretrovirinae, and we highlight whether they have been described or suspected to be transmitted through breast milk, covering various species. We also elaborate on the production and composition of breast milk and discuss potential entry sites of exogenous mammalian retroviruses during oral transmission.

Walk on the wild side: SIV infection in African non-human primate hosts-from the field to the laboratory

HIV emerged following cross-species transmissions of simian immunodeficiency viruses (SIVs) that naturally infect non-human primates (NHPs) from Africa. While HIV replication and CD4⁺ T-cell depletion lead to increased gut permeability, microbial translocation, chronic immune activation, and systemic inflammation, the natural hosts of SIVs generally avoid these deleterious consequences when infected with their species-specific SIVs and do not progress to AIDS despite persistent lifelong high viremia due to long-term coevolution with their SIV pathogens. The benign course of natural SIV infection in the natural hosts is in stark contrast to the experimental SIV infection of Asian macaques, which progresses to simian AIDS. The mechanisms of non-pathogenic SIV infections are studied mainly in African green monkeys, sooty mangabeys, and mandrills, while progressing SIV infection is experimentally modeled in macaques: rhesus macaques, pigtailed macaques, and cynomolgus macaques. Here, we focus on the distinctive features of SIV infection in natural hosts, particularly (1): the superior healing properties of the intestinal mucosa, which enable them to maintain the integrity of the gut barrier and prevent microbial translocation, thus avoiding excessive/pathologic immune activation and inflammation usually perpetrated by the leaking of the microbial products into the circulation; (2) the gut microbiome, the disruption of which is an important factor in some inflammatory diseases, yet not completely understood in the course of lentiviral infection; (3) cell population shifts resulting in target cell restriction (downregulation of CD4 or CCR5 surface molecules that bind to SIV), control of viral replication in the lymph nodes (expansion of natural killer cells), and anti-inflammatory effects in the gut (NKG2a/c⁺ CD8⁺ T cells); and (4) the genes and biological pathways that can shape genetic adaptations to viral pathogens and are associated with the non-pathogenic outcome of the natural SIV infection. Deciphering the protective mechanisms against SIV disease progression to immunodeficiency, which have been established through long-term coevolution between the natural hosts and their species-specific SIVs, may prompt the development of novel therapeutic interventions, such as drugs that can control gut inflammation, enhance gut healing capacities, or modulate the gut microbiome. These developments can go beyond HIV infection and open up large avenues for correcting gut damage, which is common in many diseases.

NK cell spatial dynamics and IgA responses in gut-associated lymphoid tissues during SIV infections

HIV infection induces tissue damage including lymph node (LN) fibrosis and intestinal epithelial barrier disruption leading to bacterial translocation and systemic inflammation. Natural hosts of SIV, such as African Green Monkeys (AGM), do not display tissue damage despite high viral load in blood and intestinal mucosa. AGM mount a NK cell-mediated control of SIVagm replication in peripheral LN. We analyzed if NK cells also control SIVagm in mesenteric (mes) LN and if this has an impact on gut humoral responses and the production of IgA known for their anti-inflammatory role in the gut. We show that CXCR5 + NK cell frequencies increase in mesLN upon SIVagm infection and that NK cells migrate into and control viral replication in B cell follicles (BCF) of mesLN. The proportion of IgA+ memory B cells were increased in mesLN during SIVagm infection in contrast to SIVmac infection. Total IgA levels in gut remained normal during SIVagm infection, while strongly decreased in intestine of chronically SIVmac-infected macaques. Our data suggest an indirect impact of NK cell-mediated viral control in mesLN during SIVagm infection on preserved BCF function and IgA production in intestinal tissues.

Differences between pathogenic and non-pathogenic SIV infections are investigated, in terms of NK cell location, function and IgA responses in gut associated lymphoid tissues (mesenteric lymph nodes, jejunum, ileon, colon).

Challenges in the management of HIV infection: update on the role of probiotic supplementation as a possible complementary therapeutic strategy for cART treated people living with HIV/AIDS

Introduction: Recent insights show that gut-mucosal immunity and intestinal microbiota play a key role in the pathogenesis of HIV infection. Alterations in the composition of intestinal flora (dysbiosis) could be associated with an impaired intestinal epithelium barrier activity and an impaired mucosal immunity function, significantly contributing to microbial translocation which is considered a major driver of chronic immune activation. Areas covered: This article provides an overview on the novel trends in probiotic therapy application. A particular emphasis is addressed to the importance of probiotics as a novel strategy to attenuate or prevent gastrointestinal involvement and to improve gut-mucosal immunity in HIV-infected subjects. Therefore, opportunities, limits and methodological criticalities of supplementation with probiotic therapy are considered and analyzed. Expert opinion: Use of probiotics is emerging as a novel strategy to manage dysbiosis and gut-mucosal impairment, to reduce immune activation and to limit a number of non-AIDS-related disorders. However, despite the growing use of probiotic therapy, mechanisms by which oral bacteria intake exhibits its effects are strain-related and disease-specific, hence clinicians need to take these two factors into consideration when suggesting probiotic supplementation to HIV-infected patients.

Marginal Effects of Systemic CCR5 Blockade with Maraviroc on Oral Simian Immunodeficiency Virus Transmission to Infant Macaques

Current approaches do not eliminate all human immunodeficiency virus type 1 (HIV-1) maternal-to-infant transmissions (MTIT); new prevention paradigms might help avert new infections. We administered maraviroc (MVC) to rhesus macaques (RMs) to block CCR5-mediated entry, followed by repeated oral exposure of a CCR5-dependent clone of simian immunodeficiency virus (SIV) mac251 (SIVmac766). MVC significantly blocked the CCR5 coreceptor in peripheral blood mononuclear cells and tissue cells. All control animals and 60% of MVC-treated infant RMs became infected by the 6th challenge, with no significant difference between the number of exposures (P = 0.15). At the time of viral exposures, MVC plasma and tissue (including tonsil) concentrations were within the range seen in humans receiving MVC as a therapeutic. Both treated and control RMs were infected with only a single transmitted/founder variant, consistent with the dose of virus typical of HIV-1 infection. The uninfected RMs expressed the lowest levels of CCR5 on the CD4+ T cells. Ramp-up viremia was significantly delayed (P = 0.05) in the MVC-treated RMs, yet peak and postpeak viral loads were similar in treated and control RMs. In conclusion, in spite of apparent effective CCR5 blockade in infant RMs, MVC had a marginal impact on acquisition and only a minimal impact on the postinfection delay of viremia following oral SIV infection. Newly developed, more effective CCR5 blockers may have a more dramatic impact on oral SIV transmission than MVC.IMPORTANCE We have previously suggested that the very low levels of simian immunodeficiency virus (SIV) maternal-to-infant transmissions (MTIT) in African nonhuman primates that are natural hosts of SIVs are due to a low availability of target cells (CCR5+ CD4+ T cells) in the oral mucosa of the infants, rather than maternal and milk factors. To confirm this new MTIT paradigm, we performed a proof-of-concept study in which we therapeutically blocked CCR5 with maraviroc (MVC) and orally exposed MVC-treated and naive infant rhesus macaques to SIV. MVC had only a marginal effect on oral SIV transmission. However, the observation that the infant RMs that remained uninfected at the completion of the study, after 6 repeated viral challenges, had the lowest CCR5 expression on the CD4+ T cells prior to the MVC treatment appears to confirm our hypothesis, also suggesting that the partial effect of MVC is due to a limited efficacy of the drug. New, more effective CCR5 inhibitors may have a better effect in preventing SIV and HIV transmission.

CCR5 Revisited: How Mechanisms of HIV Entry Govern AIDS Pathogenesis

The chemokine receptor CCR5 has been the focus of intensive studies since its role as a coreceptor for HIV entry was discovered in 1996. These studies lead to the development of small molecular drugs targeting CCR5, with maraviroc becoming in 2007 the first clinically approved chemokine receptor inhibitor. More recently, the apparent HIV cure in a patient transplanted with hematopoietic stem cells devoid of functional CCR5 rekindled the interest for inactivating CCR5 through gene therapy and pharmacological approaches. Fundamental research on CCR5 has also been boosted by key advances in the field of G-protein coupled receptor research, with the realization that CCR5 adopts a variety of conformations, and that only a subset of these conformations may be targeted by chemokine ligands. In addition, recent genetic and pathogenesis studies have emphasized the central role of CCR5 expression levels in determining the risk of HIV and SIV acquisition and disease progression. In this article, we propose to review the key properties of CCR5 that account for its central role in HIV pathogenesis, with a focus on mechanisms that regulate CCR5 expression, conformation, and interaction with HIV envelope glycoproteins.

What pediatric nonprogressors and natural SIV hosts teach us about HIV

A subset of HIV-infected children shows an unusually benign course of infection that recapitulates key immunological aspects of the natural SIV infection of sooty mangabeys (Muenchhoff et al.).

Modulation of Tryptophan/Serotonin Pathway by Probiotic Supplementation in Human Immunodeficiency Virus-Positive Patients: Preliminary Results of a New Study Approach

BACKGROUND

To date, no data are available regarding the effects of probiotics on the pathway of tryptophan/serotonin metabolism among human immunodeficiency virus (HIV) 1-infected individuals. Because a condition of dysbiosis might be responsible for the altered use of tryptophan described in this population, the aim of this study was to investigate the link between probiotic supplementation and serotonin levels in combined antiretroviral therapy-treated patients and the subsistence of an interplay with inflammation.

METHODS

We conducted a pilot study that included 8 HIV-positive subjects. We collected blood and fecal samples before and after 6 months of probiotic supplementation, to measure the level of serotonin in serum and tryptophan in stool, the expression of CD38 and HLA-DR on peripheral CD4+ T lymphocytes (as immune activation markers), the expression of indoleamine 2,3-dioxygenase 1 messenger RNA (mRNA) and IFN-γ mRNA (as markers of tryptophan metabolism and systemic inflammation).

RESULTS

After probiotic supplementation, we observed a significant increase in concentration of serum serotonin (P = .008) and a decreased level of tryptophan in plasma. Moreover, a significant reduction in CD38 and HLA-DR expression on the surface of peripheral CD4+ T cells (P = .008) and a reduced expression of indoleamine 2,3-dioxygenase 1 mRNA on peripheral blood mononuclear cells (P = .04) were observed.

CONCLUSIONS

Considering that this probiotic (Vivomixx® in EU; Visbiome® in USA) has an influence on tryptophan metabolism, larger studies on this topic are needed.

Characterization of Simian Immunodeficiency Virus Variants Anatomically Compartmentalized in Plasma and Milk in Chronically Infected African Green Monkeys

Unlike human immunodeficiency virus type 1 (HIV-1)-infected humans, African-origin, natural simian immunodeficiency virus (SIV) hosts, such as African green monkeys (AGMs), sustain nonpathogenic SIV infections and rarely vertically transmit SIV to their infants. Interestingly, chronically SIV-infected AGMs have anatomically compartmentalized SIV variants in plasma and milk, whereas humans and SIV-infected rhesus monkeys (RMs), Asian-origin nonnatural SIV hosts, do not exhibit this compartmentalization. Thus, it is possible that AGM SIV populations in milk have unique phenotypic features that contribute to the low postnatal transmission rates observed in this natural host species. In this study, we explored this possibility by characterizing the infectivity, tropism, and neutralization susceptibility of plasma and milk SIVsab env variants isolated from chronically SIVsab92018ivTF-infected AGMs. AGM plasma and milk SIVsab env pseudovirus variants exhibited similar infectivities, neutralization susceptibilities to autologous and heterologous plasma, and chemokine coreceptor usages for cell entry, suggesting similar abilities to initiate infection in a new host. We also assessed the cytokine milieu in SIV-infected AGM milk and compared it to that of SIV-infected RMs. MIP-1β, granulocyte colony-stimulating factor (G-CSF), interleukin-12/23 (IL-12/23), and IL-13 trended significantly higher in SIV-infected AGM milk than in that of RMs, while IL-18 and IL-6 trended significantly higher in SIV-infected RM milk than in that of AGMs. Taken together, our findings imply that nonviral maternal factors, such as the cytokine milieu, rather than unique characteristics of SIV populations in the milk contribute to the low postnatal transmission rates observed in AGMs.

IMPORTANCE

Due to the ongoing global incidence of pediatric HIV-1 infections, including many that occur via breastfeeding, development of effective vaccine strategies capable of preventing vertical HIV transmission through breastfeeding remains an important goal. Unlike HIV-1-infected humans, African green monkeys (AGMs), the natural SIV host species, sustain nonpathogenic SIV infections, rarely transmit the virus postnatally to their infants, and exhibit anatomically compartmentalized SIV populations in milk and plasma. Identifying unique features of the anatomically compartmentalized milk SIV populations could enhance our understanding of how AGMs may have evolved to avoid transmission through breastfeeding. While this study identified limited phenotypic distinctions between AGM plasma and milk SIV populations, potential differences in milk cytokine profiles of natural and nonnatural SIV hosts were observed. These findings imply the potential importance of nonviral factors in natural SIV host species, such as innate SIV/HIV immune factors in milk, as a means of naturally preventing vertical transmission.

Non-human primates in HIV research: Achievements, limits and alternatives

An ideal model for HIV-1 research is still unavailable. However, infection of non-human primates (NHP), such as macaques, with Simian Immunodeficiency Virus (SIV) recapitulates most virological, immunological and clinical hallmarks of HIV infection in humans. It has become the most suitable model to study the mechanisms of transmission and physiopathology of HIV/AIDS. On the other hand, natural hosts of SIV, such as African green monkeys and sooty mangabeys that when infected do not progress to AIDS, represent an excellent model to elucidate the mechanisms involved in the capacity of controlling inflammation and disease progression. The use of NHP-SIV models has indeed enriched our knowledge in the fields of: i) viral transmission and viral reservoirs, ii) early immune responses, iii) host cell-virus interactions in tissues, iv) AIDS pathogenesis, v) virulence factors, vi) prevention and vii) drug development. The possibility to control many variables during experimental SIV infection, together with the resemblance between SIV and HIV infections, make the NHP model the most appropriate, so far, for HIV/AIDS research. Nonetheless, some limitations in using these models have to be considered. Alternative models for HIV/AIDS research, such as humanized mice and recombinant forms of HIV-SIV viruses (SHIV) for NHP infection, have been developed. The improvement of SHIV viruses that mimic even better the natural history of HIV infection and of humanized mice that develop a greater variety of human immune cell lineages, is ongoing. None of these models is perfect, but they allow contributing to the progress in managing or preventing HIV infection.

The well-tempered SIV infection: Pathogenesis of SIV infection in natural hosts in the wild, with emphasis on virus transmission and early events post-infection that may contribute to protection from disease progression

African NHPs are infected by over 40 different simian immunodeficiency viruses. These viruses have coevolved with their hosts for long periods of time and, unlike HIV in humans, infection does not generally lead to disease progression. Chronic viral replication is maintained for the natural lifespan of the host, without loss of overall immune function. Lack of disease progression is not correlated with transmission, as SIV infection is highly prevalent in many African NHP species in the wild. The exact mechanisms by which these natural hosts of SIV avoid disease progression are still unclear, but a number of factors might play a role, including: (i) avoidance of microbial translocation from the gut lumen by preventing or repairing damage to the gut epithelium; (ii) control of immune activation and apoptosis following infection; (iii) establishment of an anti-inflammatory response that resolves chronic inflammation; (iv) maintenance of homeostasis of various immune cell populations, including NK cells, monocytes/macrophages, dendritic cells, Tregs, Th17 T-cells, and γδ T-cells; (v) restriction of CCR5 availability at mucosal sites; (vi) preservation of T-cell function associated with down-regulation of CD4 receptor. Some of these mechanisms might also be involved in protection of natural hosts from mother-to-infant SIV transmission during breastfeeding. The difficulty of performing invasive studies in the wild has prohibited investigation of the exact events surrounding transmission in natural hosts. Increased understanding of the mechanisms of SIV transmission in natural hosts, and of the early events post-transmission which may contribute to avoidance of disease progression, along with better comprehension of the factors involved in protection from SIV breastfeeding transmission in the natural hosts, could prove invaluable for the development of new prevention strategies for HIV.

Rapid Development of gp120-Focused Neutralizing B Cell Responses during Acute Simian Immunodeficiency Virus Infection of African Green Monkeys

The initial phases of acute human immunodeficiency virus type 1 (HIV-1) infection may be critical for development of effective envelope (Env)-specific antibodies capable of impeding the establishment of the latent pool of HIV-1-infected CD4(+) T cells, preventing virus-induced immune hyperactivation to limit disease progression and blocking vertical virus transmission. However, the initial systemic HIV-1 Env-specific antibody response targets gp41 epitopes and fails to control acute-phase viremia. African-origin, natural simian immunodeficiency virus (SIV) hosts do not typically progress to AIDS and rarely postnatally transmit virus to their infants, despite high milk viral loads. Conversely, SIV-infected rhesus macaques (RMs), Asian-origin nonnatural SIV hosts, sustain pathogenic SIV infections and exhibit higher rates of postnatal virus transmission. In this study, of acute SIV infection, we compared the initial systemic Env-specific B cell responses of AGMs and RMs in order to probe potential factors influencing the lack of disease progression observed in AGMs. AGMs developed higher-magnitude plasma gp120-specific IgA and IgG responses than RMs, whereas RMs developed more robust gp140-directed IgG responses. These gp120-focused antibody responses were accompanied by rapid autologous neutralizing responses during acute SIV infection in AGMs compared to RMs. Moreover, acute SIV infection elicited a higher number of circulating Env-specific memory B cells in peripheral blood of AGMs than in the blood of RMs. These findings indicate that AGMs have initial systemic Env-specific B cell responses to SIV infection distinct from those of a nonnatural SIV host, resulting in more functional SIV-specific humoral responses, which may be involved in impairing pathogenic disease progression and minimizing postnatal transmission.

IMPORTANCE

Due to the worldwide prevalence of HIV-1 infections, development of a vaccine to prevent infection or limit the viral reservoir remains an important goal. HIV-1-infected humans, as well as SIV-infected nonnatural SIV hosts, develop pathogenic infections and readily transmit the virus to their infants. Conversely, natural SIV hosts do not develop pathogenic infections and rarely transmit the virus to their infants. The immunologic factors contributing to these favorable outcomes in natural SIV hosts could prove invaluable for directing HIV-1 vaccine and therapy design. This study identified distinctions in the specificity and function of the initial systemic SIV envelope-specific B cell response that developed during acute SIV infection in natural and nonnatural SIV host species. Identification of distinct acute B cell responses in natural SIV hosts may inform vaccine strategies seeking to elicit similar responses prior to or during the initial phases of acute HIV-1 infection.

Immunity to HIV in Early Life

The developing immune system is adapted to the exposure to a plethora of pathogenic and non-pathogenic antigens encountered in utero and after birth, requiring a fine balance between protective immunity and immune tolerance. In early stages of life, this tolerogenic state of the innate and adaptive immune system and the lack of immunological memory render the host more susceptible to infectious pathogens like HIV. HIV pathogenesis is different in children, compared to adults, with more rapid disease progression and a substantial lack of control of viremia compared to adults. Plasma viral load remains high during infancy and only declines gradually over several years in line with immune maturation, even in rare cases where children maintain normal CD4 T-lymphocyte counts for several years without antiretroviral therapy (ART). These pediatric slow progressors also typically show low levels of immune activation despite persistently high viremia, resembling the phenotype of natural hosts of SIV infection. The lack of immunological memory places the fetus and the newborn at higher risk of infections; however, it may also provide an opportunity for unique interventions. Frequencies of central memory CD4+ T-lymphocytes, one of the main cellular reservoirs of HIV, are very low in the newborn child, so immediate ART could prevent the establishment of persistent viral reservoirs and result in "functional cure." However, as recently demonstrated in the case report of the "Mississippi child" who experienced viral rebound after more than 2 years off ART, additional immunomodulatory strategies might be required for sustained viral suppression after ART cessation. In this review, we discuss the interactions between HIV and the developing immune system in children and the potential implications for therapeutic and prophylactic interventions.

Clinicopathologic characteristics, prevalence, and risk factors of spontaneous diabetes in sooty mangabeys (Cercocebus atys)

In 2008, clinical observations in our colony of sooty mangabeys (Cercocebus atys) suggested a high frequency of type 2 diabetes. Postmortem studies of diabetic animals revealed dense amyloid deposits in pancreatic islets. To investigate these findings, we screened our colony (97 male mangabeys; 99 female mangabeys) for the disease from 2008 to 2012. The overall prevalence of diabetes was 11% and of prediabetes was 7%, which is nearly double that reported for other primate species (less than 6%). Fructosamine and triglyceride levels were the best indicators of diabetes; total cholesterol and glycated hemoglobin were not associated with disease. Increasing age was a significant risk factor: prevalence increased from 0% in infants, juveniles, and young adults to 11% in adults and 19% in geriatric mangabeys. Sex, medroxyprogesterone acetate exposure, and SIV status were unrelated to disease. Weight was marginally higher in prediabetics, but body condition did not indicate obesity. Of the 49 mangabeys that were necropsied after clinical euthanasia or death from natural causes, 22 were diabetic; all 22 animals demonstrated pancreatic amyloid, and most had more than 75% of islets replaced with amyloid. We conclude that type 2 diabetes is more common in mangabeys than in other primate species. Diabetes in mangabeys has some unusual pathologic characteristics, including the absence of altered cholesterol levels and glycated hemoglobin but a robust association of pancreatic insular amyloidosis with clinical diabetes. Future research will examine the genetic basis of mangabey diabetes and evaluate additional diagnostic tools using imaging and serum markers.

Factors associated with siman immunodeficiency virus transmission in a natural African nonhuman primate host in the wild

African green monkeys (AGMs) are naturally infected with simian immunodeficiency virus (SIV) at high prevalence levels and do not progress to AIDS. Sexual transmission is the main transmission route in AGM, while mother-to-infant transmission (MTIT) is negligible. We investigated SIV transmission in wild AGMs to assess whether or not high SIV prevalence is due to differences in mucosal permissivity to SIV (i.e., whether the genetic bottleneck of viral transmission reported in humans and macaques is also observed in AGMs in the wild). We tested 121 sabaeus AGMs (Chlorocebus sabaeus) from the Gambia and found that 53 were SIV infected (44%). By combining serology and viral load quantitation, we identified 4 acutely infected AGMs, in which we assessed the diversity of the quasispecies by single-genome amplification (SGA) and documented that a single virus variant established the infections. We thus show that natural SIV transmission in the wild is associated with a genetic bottleneck similar to that described for mucosal human immunodeficiency virus (HIV) transmission in humans. Flow cytometry assessment of the immune cell populations did not identify major differences between infected and uninfected AGM. The expression of the SIV coreceptor CCR5 on CD4+ T cells dramatically increased in adults, being higher in infected than in uninfected infant and juvenile AGMs. Thus, the limited SIV MTIT in natural hosts appears to be due to low target cell availability in newborns and infants, which supports HIV MTIT prevention strategies aimed at limiting the target cells at mucosal sites. Combined, (i) the extremely high prevalence in sexually active AGMs, (ii) the very efficient SIV transmission in the wild, and (iii) the existence of a fraction of multiparous females that remain uninfected in spite of massive exposure to SIV identify wild AGMs as an acceptable model of exposed, uninfected individuals.

IMPORTANCE

We report an extensive analysis of the natural history of SIVagm infection in its sabaeus monkey host, the African green monkey species endemic to West Africa. Virtually no study has investigated the natural history of SIV infection in the wild. The novelty of our approach is that we report for the first time that SIV infection has no discernible impact on the major immune cell populations in natural hosts, thus confirming the nonpathogenic nature of SIV infection in the wild. We also focused on the correlates of SIV transmission, and we report, also for the first time, that SIV transmission in the wild is characterized by a major genetic bottleneck, similar to that described for HIV-1 transmission in humans. Finally, we report here that the restriction of target cell availability is a major correlate of the lack of SIV transmission to the offspring in natural hosts of SIVs.

Target cell availability, rather than breast milk factors, dictates mother-to-infant transmission of SIV in sooty mangabeys and rhesus macaques

Mother-to-infant transmission (MTIT) of HIV is a serious global health concern, with over 300,000 children newly infected in 2011. SIV infection of rhesus macaques (RMs) results in similar rates of MTIT to that of HIV in humans. In contrast, SIV infection of sooty mangabeys (SMs) rarely results in MTIT. The mechanisms underlying protection from MTIT in SMs are unknown. In this study we tested the hypotheses that breast milk factors and/or target cell availability dictate the rate of MTIT in RMs (transmitters) and SMs (non-transmitters). We measured viral loads (cell-free and cell-associated), levels of immune mediators, and the ability to inhibit SIV infection in vitro in milk obtained from lactating RMs and SMs. In addition, we assessed the levels of target cells (CD4+CCR5+ T cells) in gastrointestinal and lymphoid tissues, including those relevant to breastfeeding transmission, as well as peripheral blood from uninfected RM and SM infants. We found that frequently-transmitting RMs did not have higher levels of cell-free or cell-associated viral loads in milk compared to rarely-transmitting SMs. Milk from both RMs and SMs moderately inhibited in vitro SIV infection, and presence of the examined immune mediators in these two species did not readily explain the differential rates of transmission. Importantly, we found that the percentage of CD4+CCR5+ T cells was significantly lower in all tissues in infant SMs as compared to infant RMs despite robust levels of CD4+ T cell proliferation in both species. The difference between the frequently-transmitting RMs and rarely-transmitting SMs was most pronounced in CD4+ memory T cells in the spleen, jejunum, and colon as well as in central and effector memory CD4+ T cells in the peripheral blood. We propose that limited availability of SIV target cells in infant SMs represents a key evolutionary adaptation to reduce the risk of MTIT in SIV-infected SMs.

Currently 2.5 million children are infected with HIV, largely as a result of mother-to-child transmission, and there is no effective vaccine or cure. Studies of Simian Immunodeficiency Virus (SIV) infection of nonhuman primate species termed “natural hosts” have shown that mother-to-infant transmission of SIV in these animals is rare. Natural hosts are African monkey species that are naturally infected with SIV in the wild but do not develop AIDS. We sought to understand the mechanism by which natural hosts are protected from mother-to-infant transmission of SIV, aiming to translate our findings into novel strategies to prevent perinatal HIV infection. We found that natural host sooty mangabey infants have extremely low levels of target cells for SIV infection in lymphoid and gastrointestinal tissues. Direct comparison of infant sooty mangabeys and infant rhesus macaques (non-natural host species with high SIV transmission rates) confirmed that natural hosts have significantly lower levels of SIV target cells compared with non-natural hosts. Analysis of the breast milk of sooty mangabeys and rhesus macaques revealed similar levels of virus and ability to inhibit SIV infection. Our study provides evidence for target cell restriction as the main mechanism of protection from mother-to-infant SIV transmission in natural hosts.

The oral mucosa immune environment and oral transmission of HIV/SIV

The global spread of human immunodeficiency virus (HIV) is dependent on the ability of this virus to efficiently cross from one host to the next by traversing a mucosal membrane. Unraveling how mucosal exposure of HIV results in systemic infection is critical for the development of effective therapeutic strategies. This review focuses on understanding the immune events associated with the oral route of transmission (via breastfeeding or sexual oral intercourse), which occurs across the oral and/or gastrointestinal mucosa. Studies in both humans and simian immunodeficiency virus (SIV) monkey models have identified viral changes and immune events associated with oral HIV/SIV exposure. This review covers our current knowledge of HIV oral transmission in both infants and adults, the use of SIV models in understanding early immune events, oral immune factors that modulate HIV/SIV susceptibility (including mucosal inflammation), and interventions that may impact oral HIV transmission rates. Understanding the factors that influence oral HIV transmission will provide the foundation for developing immune therapeutic and vaccine strategies that can protect both infants and adults from oral HIV transmission.

Immunology of pediatric HIV infection

Most infants born to human immunodeficiency virus (HIV)-infected women escape HIV infection. Infants evade infection despite an immature immune system and, in the case of breastfeeding, prolonged repetitive exposure. If infants become infected, the course of their infection and response to treatment differs dramatically depending upon the timing (in utero, intrapartum, or during breastfeeding) and potentially the route of their infection. Perinatally acquired HIV infection occurs during a critical window of immune development. HIV's perturbation of this dynamic process may account for the striking age-dependent differences in HIV disease progression. HIV infection also profoundly disrupts the maternal immune system upon which infants rely for protection and immune instruction. Therefore, it is not surprising that infants who escape HIV infection still suffer adverse effects. In this review, we highlight the unique aspects of pediatric HIV transmission and pathogenesis with a focus on mechanisms by which HIV infection during immune ontogeny may allow discovery of key elements for protection and control from HIV.

Lack of B cell dysfunction is associated with functional, gp120-dominant antibody responses in breast milk of simian immunodeficiency virus-infected African green monkeys

The design of an effective vaccine to reduce the incidence of mother-to-child transmission (MTCT) of human immunodeficiency virus (HIV) via breastfeeding will require identification of protective immune responses that block postnatal virus acquisition. Natural hosts of simian immunodeficiency virus (SIV) sustain nonpathogenic infection and rarely transmit the virus to their infants despite high milk virus RNA loads. This is in contrast to HIV-infected women and SIV-infected rhesus macaques (RhMs), nonnatural hosts which exhibit higher rates of postnatal virus transmission. In this study, we compared the systemic and mucosal B cell responses of lactating, SIV-infected African green monkeys (AGMs), a natural host species, to that of SIV-infected RhMs and HIV-infected women. AGMs did not demonstrate hypergammaglobulinemia or accumulate circulating memory B cells during chronic SIV infection. Moreover, the milk of SIV-infected AGMs contained higher proportions of naive B cells than RhMs. Interestingly, AGMs exhibited robust milk and plasma Env binding antibody responses that were one to two logs higher than those in RhMs and humans and demonstrated autologous neutralizing responses in milk at 1 year postinfection. Furthermore, the plasma and milk Env gp120-binding antibody responses were equivalent to or predominant over Env gp140-binding antibody responses in AGMs, in contrast to that in RhMs and humans. The strong gp120-specific, functional antibody responses in the milk of SIV-infected AGMs may contribute to the rarity of postnatal transmission observed in natural SIV hosts.

Transient compartmentalization of simian immunodeficiency virus variants in the breast milk of african green monkeys

Natural hosts of simian immunodeficiency virus (SIV), African green monkeys (AGMs), rarely transmit SIV via breast-feeding. In order to examine the genetic diversity of breast milk SIV variants in this limited-transmission setting, we performed phylogenetic analysis on envelope sequences of milk and plasma SIV variants of AGMs. Low-diversity milk virus populations were compartmentalized from that in plasma. However, this compartmentalization was transient, as the milk virus lineages did not persist longitudinally.

Nonpathogenic simian immunodeficiency virus infections

The simian immunodeficiency viruses (SIVs) are a diverse group of viruses that naturally infect a wide range of African primates, including African green monkeys (AGMs) and sooty mangabey monkeys (SMs). Although natural infection is widespread in feral populations of AGMs and SMs, this infection generally does not result in immunodeficiency. However, experimental inoculation of Asian macaques results in an immunodeficiency syndrome remarkably similar to human AIDS. Thus, natural nonprogressive SIV infections appear to represent an evolutionary adaptation between these animals and their primate lentiviruses. Curiously, these animals maintain robust virus replication but have evolved strategies to avoid disease progression. Adaptations observed in these primates include phenotypic changes to CD4(+) T cells, limited chronic immune activation, and altered mucosal immunity. It is probable that these animals have achieved a unique balance between T-cell renewal and proliferation and loss through activation-induced apoptosis, and virus-induced cell death. A clearer understanding of the mechanisms underlying the lack of disease progression in natural hosts for SIV infection should therefore yield insights into the pathogenesis of AIDS and may inform vaccine design.

SIVagm infection in wild African green monkeys from South Africa: epidemiology, natural history, and evolutionary considerations

Pathogenesis studies of SIV infection have not been performed to date in wild monkeys due to difficulty in collecting and storing samples on site and the lack of analytical reagents covering the extensive SIV diversity. We performed a large scale study of molecular epidemiology and natural history of SIVagm infection in 225 free-ranging AGMs from multiple locations in South Africa. SIV prevalence (established by sequencing pol, env, and gag) varied dramatically between infant/juvenile (7%) and adult animals (68%) (p<0.0001), and between adult females (78%) and males (57%). Phylogenetic analyses revealed an extensive genetic diversity, including frequent recombination events. Some AGMs harbored epidemiologically linked viruses. Viruses infecting AGMs in the Free State, which are separated from those on the coastal side by the Drakensberg Mountains, formed a separate cluster in the phylogenetic trees; this observation supports a long standing presence of SIV in AGMs, at least from the time of their speciation to their Plio-Pleistocene migration. Specific primers/probes were synthesized based on the pol sequence data and viral loads (VLs) were quantified. VLs were of 10(4)-10(6) RNA copies/ml, in the range of those observed in experimentally-infected monkeys, validating the experimental approaches in natural hosts. VLs were significantly higher (10(7)-10(8) RNA copies/ml) in 10 AGMs diagnosed as acutely infected based on SIV seronegativity (Fiebig II), which suggests a very active transmission of SIVagm in the wild. Neither cytokine levels (as biomarkers of immune activation) nor sCD14 levels (a biomarker of microbial translocation) were different between SIV-infected and SIV-uninfected monkeys. This complex algorithm combining sequencing and phylogeny, VL quantification, serology, and testing of surrogate markers of microbial translocation and immune activation permits a systematic investigation of the epidemiology, viral diversity and natural history of SIV infection in wild African natural hosts.

New strain of simian immunodeficiency virus identified in wild-born chimpanzees from central Africa

Studies of primate lentiviruses continue to provide information about the evolution of simian immunodeficiency viruses (SIVs) and the origin and emergence of HIV since chimpanzees in west–central Africa (Pan troglodytes troglodytes) were recognized as the reservoir of SIVcpzPtt viruses, which have been related phylogenetically to HIV-1. Using in-house peptide ELISAs to study SIV prevalence, we tested 104 wild-born captive chimpanzees from Gabon and Congo. We identified two new cases of SIVcpz infection in Gabon and characterized a new SIVcpz strain, SIVcpzPtt-Gab4. The complete sequence (9093 bp) was obtained by a PCR-based ‘genome walking’ approach to generate 17 overlapping fragments. Phylogenetic analyses of separated genes (gag, pol-vif and env-nef) showed that SIVcpzPtt-Gab4 is closely related to SIVcpzPtt-Gab1 and SIVcpzPtt-Gab2. No significant variation in viral load was observed during 3 years of follow-up, but a significantly lower CD4+ T cells count was found in infected than in uninfected chimpanzees (p<0.05). No clinical symptoms of SIV infection were observed in the SIV-positive chimpanzees. Further field studies with non-invasive methods are needed to determine the prevalence, geographic distribution, species association, and natural history of SIVcpz strains in the chimpanzee habitat in Gabon.

Natural SIV hosts: showing AIDS the door

Many species of African nonhuman primates are naturally infected with simian immunodeficiency viruses (SIVs) in the wild and in captivity. In contrast to HIV-infected humans, these natural SIV hosts typically do not develop AIDS, despite chronic infection with a highly replicating virus. In this Review, we discuss the most recent advances on the mechanisms of protection from disease progression in natural SIV hosts, with emphasis on how they differ from pathogenic HIV/SIV infections of humans and rhesus macaques. These mechanisms include: (i) resolution of immune activation after acute infection, (ii) restricted pattern of target cell infection, and (iii) protection from mother-to-infant transmission. We highlight the areas that should be pursued in future studies, focusing on potential applications for the treatment and prevention of HIV infection.

Mucosal simian immunodeficiency virus transmission in African green monkeys: susceptibility to infection is proportional to target cell availability at mucosal sites

African green monkeys (AGMs) are naturally infected with a simian immunodeficiency virus (SIVagm) that is nonpathogenic in its host. Although SIVagm is common and widespread, little is known about the mechanisms that govern its transmission. Since the earliest virus-host interactions may provide key insights into the nonpathogenic phenotype of SIVagm, we developed a mucosal transmission model for this virus. Using plasma from an acutely infected AGM as the virus inoculum, we exposed adult and juvenile AGMs, as well as pigtailed macaques (PTMs) as a nonnatural host control, by mucosal routes to increasing titers of virus and compared the doses needed to establish a productive infection. Four juvenile and four adult AGMs as well as two PTMs were intrarectally (IR) exposed, while two additional adult female AGMs were intravaginally (IVAG) exposed. No animal became infected following exposure to 10(5) RNA copies. Both PTMs but none of the AGMs became infected following exposure to 10(6) RNA copies. Finally, all adult AGMs and two of the four juvenile AGMs became infected following exposure to 10(7) RNA copies, acquiring either one (2 IR infected juveniles, 1 IR infected adult, 2 IVAG infected adults) or two (3 IR infected adults) transmitted founder viruses. These results were consistent with immunophenotypic data, which revealed a significant correlation between the percentage of CD4(+) T cells expressing CCR5 in the mucosa and the susceptibility to infection, in terms of both the viral dose and the numbers of transmitted founder viruses. Moreover, studies of uninfected AGMs showed that the fraction of CCR5-expressing CD4(+) T cells increased significantly with age. These results indicate that (i) AGMs are readily infected with SIVagm by both intrarectal and intravaginal routes, (ii) susceptibility to infection is proportional to the number of available CCR5(+) CD4(+) target cells in the mucosa, and (iii) the paucity of CCR5(+) CD4(+) target cells in infant and juvenile AGMs may explain the near absence of vertical transmission.