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Combination of Antibodies Prevent HIV-Infection in Monkeys
  "Post-exposure prophylaxis with human monoclonal antibodies prevented SHIV89.6P infection or disease in neonatal macaques"
Excerpts from published study results:
Using post-exposure immunoprophylaxis in a primate model, we have developed a novel strategy to prevent mother-to-child transmission of HIV. Neonatal monkeys were challenged orally with SHIV89.6P and infused with a quadruple mAb combination post-exposure. Protection from systemic infection was achieved in two of four animals; a third infant had neither CD4 T cell decline nor signs of disease. Another monkey had a minor and delayed drop in CD4 T cells. Long-term follow-up of the two mAb-treated animals that had no viremia early on showed no resurgence of virus, even after the human mAbs had been cleared (by day 120). Therefore, post-exposure passive immunization provided potent protection from SHIV89.6P infection and disease.
At the end of 2000, 1.4 million children worldwide were infected with HIV; most infections resulted from maternal transmission and most occurred in Africa. HIV can be transmitted during gestation, intrapartum, or by breastfeeding. Mucosal, especially oral, exposure represents an important route of intrapartum HIV transmission. A recent study reported a 16.2% frequency of HIV transmission through breast milk; most infants became infected during the first 6 months. The benefits of perinatal antiretroviral drug prophylaxis may be lost by continued exposure to HIV through breastfeeding, as demonstrated in the long-term follow-up of the PETRA Study carried out in South Africa, Tanzania, and Uganda. Pregnant women in group A were given zidovudine and lamivudine starting from 36 weeks of gestation onward and during labor and delivery; their infants were treated for 1 week. This regimen reduced HIV transmission and infant mortality by 61% at 6 weeks; at 18 months, however, the reduction was only 26%. Women in group B were treated only intrapartum, and their infants received drugs postnatally. At 6 weeks, the relative efficacy was 36%, which was reduced to 7% at 18 months. Among breastfed infants, the differences between regimens A or B versus placebo were no longer statistically significant.
We propose to develop passive immunization with combinations of neutralizing human anti-HIV monoclonal antibodies (mAbs) against intrapartum as well as postnatal HIV transmission through breastfeeding. We have developed primate models for preclinical studies using oral challenge with chimeric simian-human immunodeficiency virus (SHIV) strains encoding HIV env in a SIV backbone. Earlier, we showed that neonatal macaques were protected completely from SHIV-vpu+ oral challenge with a triple combination of human anti-HIV mAbs, either when this was given first to pregnant dams and then to the newborns both before and after virus exposure or when it was only given to the newborns. A triple mAb combination partially protected neonatal monkeys against subsequent pathogenic SHIV89.6P oral challenge. However, to be clinically applicable for prevention of intrapartum HIV transmission, passive immunization needs to be effective when given as post-exposure prophylaxis to newborns. Such an approach effectively blocks maternal transmission of the hepatitis B virus.
To develop a more potent immunoprophylaxis regimen against SHIV89.6P challenge, four human mAbs (IgG1b12, 2G12, 2F5, and 4E10) were initially tested in vitro; complete virus neutralization was achieved. These mAbs, previously generated from HIV-1 clade B-infected individuals, are directed against conserved HIV-1 Env epitopes and, when combined, potently neutralized primary HIV clade C strains. The present study describes the use of this combination in post-exposure passive immunization. Two out of four treated macaque infants were protected from systemic infection and the other two against SHIV89. 6P-induced acute disease.
The neutralizing activities of a panel of human anti-HIV mAbs were first tested in vitro. 2G12 was most potent as single agent against SHIV89.6P in either macaque or human PBMC followed by 2F5 and 4E10. When used alone, IgG1b12 was unable to neutralize SHIV89.6P, confirming previous results. Yet, it was previously shown that IgG1b12 enhanced the neutralization potency of 2G12 and 2F5 when used in combination. The quadruple combination of IgG1b12, 2G12, 2F5, and 4E10 was more potent in vitro than the combination lacking 4E10. This latter triple combination had previously yielded partial protection of macaque neonates treated prophylactically.
Background: The majority of infants infected through maternal transmission acquire the virus during birth or postpartum through breastfeeding: mucosal exposure is considered to be a major route of infection.
Objectives: To develop passive immunization with human neutralizing monoclonal antibodies (mAbs) against mother-to-child transmission of HIV during delivery and through breastfeeding.
Design: An oral challenge model in newborn rhesus macaques mimicked peri- and postpartum virus transmission.
Methods: Neonatal rhesus macaques were challenged orally with the highly pathogenic, chimeric simian-human immunodeficiency virus SHIV89.6P and given post-exposure prophylaxis with a quadruple combination of neutralizing human mAbs, IgG1b12, 2G12, 2F5, and 4E10, directed against conserved epitopes of HIV envelope glycoproteins. Control animals were virus challenged but left untreated. All infants were followed prospectively for signs of viremia and immunodeficiency.
Results: Two out of four macaque infants treated with neutralizing mAbs showed no evidence of infection; the other two maintained normal CD4 T cell counts. In contrast, all control animals became highly viremic and had profound CD4 T cell losses; three out of four died from AIDS within 1.5-6 weeks of the challenge.
Post-exposure passive immunization can protect newborn macaques against mucosal SHIV89.6P challenge
Four newborn rhesus macaques were challenged orally with 15 AID50 of pathogenic SHIV89.6P. After 1 h, infants were infused intravenously with mAbs 2F5, 2G12, 4E10, and IgG1b12 and again 8 days later . Within the limits of the sensitive assays used, there was no evidence of infection in two animals, RKy-7 and RMy-7; no plasma viral RNA could be detected, and virus isolation at week 1 (when viremia peaked in controls) was negative (not shown). RKy-7 and RMy-7 also maintained normal CD4 T cell counts throughout (Fig. 2d,f). RJy-7 had both delayed and lower peak plasma viremia compared with untreated controls; neither a decline in CD4 T cells nor signs of disease were observed even 1 year later in this animal. RSy-7 also had a delayed peak plasma viremia; its CD4 T cell counts transiently declined, but never fell to < 500 x 10 6th cells/l, and recovered to age-appropriate levels of > 1500 x 10 6th cells/l. Four control neonates were orally exposed to SHIV89.6P but left untreated; all rapidly developed high plasma viremia levels, and virus could be isolated repeatedly from PBMC by cocultivation (not shown). All four controls had steep drops in their CD4 T cell counts by week 2.
Clinical follow-up
MAb-treated animals RJy-7, RKy-7, and RMy-7 remained healthy during more than 1 year follow-up. Treated animal RSy-7 was euthanized at 6 months of age because of intermittent diarrhea and failure to thrive. Stool cultures revealed campylobacteriosis (common in naive monkey infants with diarrhea at the same primate center). However, at necropsy, RSy-7 had no lymphoid depletion, viral RNA load was only 1563 copies/ml, and the CD4 T cell count (888 x 10 6th cells/l) was slightly low for the age of the animal. Three controls were sacrificed between day 10 and week 6 post-inoculation because of opportunistic infections. RCy-7 had generalized lymphoid depletion, necrotizing hepatitis, and encephalopathy of unknown etiology; REt-7 exhibited pancreatitis and pneumonia caused by adenovirus infection and cryptosporidiosis; RFy-7 presented with acute bacterial enteritis and lymphadenopathy; histological analysis showed disseminated cryptosporidiosis. The fourth control animal recovered from CD4 T cell loss.
Undetectable virus on follicular dendritic cells from protected animals
During chronic infection, FDC in secondary lymphoid tissues are known to trap antibody-covered HIV particles. Protected animals RKy-7 and RMy-7 were examined to determine whether they retained virus on FDC in their lymph nodes. RNA isolated from these infants' FDC at 32 and 30 weeks post-exposure, respectively, was virus-negative by RT-PCR. As positive controls, RNA from FDC isolated from viremic animals RDt-7 and RSy-7 at weeks 43 and 17 post-challenge, respectively, was positive for SHIV89.6P RNA.
We have repeatedly detected viral antigen-specific proliferative responses in the two protected animals that never showed evidence of viremia, indicating that the host may be able to eliminate a certain number of infected cells by mounting specific antiviral cellular immune responses. Therefore, passively administered neutralizing mAbs might be assisted by the active response of the host's immune system in preventing the spread of infection. Substantial cellular immune responses were generated by the two partially protected animals, in which mAb treatment prevented acute T cell loss. These results strongly suggest that passive immunotherapy could play a role as post-exposure prophylaxis for intrapartum virus transmission.
Since our primate model also mimics oral HIV transmission via infected breast milk, our results suggest that passive immunization with neutralizing human mAbs may protect neonates against milk-borne HIV transmission. In many countries, treatment with antiretroviral drugs is now standard therapy to prevent perinatal mother-to-child transmission of HIV. However, in developing countries, the benefits of perinatal drug treatment may be reduced or lost because of milk-borne virus transmission. Furthermore, the long-term effects of antiviral drugs such as single-agent nevirapine, which is being considered as drug prophylaxis against breastmilk transmission in sub-Saharan Africa (H. Coovadia, personal communication), are not yet known. These concerns may be circumvented through passive immunization with neutralizing human mAbs. Antibodies have long half-lives in vivo and are unlikely to be toxic, as indicated by the present and other studies. In a phase I clinical trial, 2G12 and 2F5 have been administered safely to HIV-infected adults and also exhibited antiviral activity.
Conclusions: Passive immunization with this quadruple neutralizing mAbs combination may represent a promising approach to prevent peri- and postnatal HIV transmission. Furthermore, the epitopes recognized by the four neutralizing mAbs are key determinants to achieve complete protection and represent important targets against which to develop active, antibody-response-based AIDS vaccines.
Our study showed that antibody binding to the four conserved epitopes recognized by IgG1b12, 2G12, 2F5, and 4E10 was sufficient to neutralize virus infectivity in vitro and achieve protection against systemic infection in two animals. We conclude that antibodies to these epitopes are not just correlates, but actual causes of immunoprotection against a pathogenic primate lentivirus. Therefore, our data have important implications for AIDS vaccine development. Although serum levels of antibodies required to neutralize the virus completely are not yet achievable with the currently available HIV immunogens, a vaccine that can induce antibodies to the four epitopes identified here by passive immunization could be promising.
AIDS 2003; 17(3):301-309
Flavia Ferrantelli et al. Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute, the Department of Medicine, Harvard Medical School
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