Waning immunity and IgG4 responses following bivalent mRNA boosting

Messenger RNA (mRNA) vaccines were highly effective against the ancestral SARS-CoV-2 strain, but the efficacy of bivalent mRNA boosters against XBB variants was substantially lower. Here, we show limited durability of neutralizing antibody (NAb) responses against XBB variants and isotype switching to immunoglobulin G4 (IgG4) responses following bivalent mRNA boosting. Bivalent mRNA boosting elicited modest XBB.1-, XBB.1.5-, and XBB.1.16-specific NAbs that waned rapidly within 3 months. In contrast, bivalent mRNA boosting induced more robust and sustained NAbs against the ancestral WA1/2020 strain, suggesting immune imprinting. Following bivalent mRNA boosting, serum antibody responses were primarily IgG2 and IgG4 responses with poor Fc functional activity. In contrast, a third monovalent mRNA immunization boosted all isotypes including IgG1 and IgG3 with robust Fc functional activity. These data show substantial immune imprinting for the ancestral spike and isotype switching to IgG4 responses following bivalent mRNA boosting, with important implications for future booster designs and boosting strategies.

Altered Cytokine Production in Human Intervillous Blood T Cells in Preeclampsia

Conventional and regulatory T cells (Treg) are dynamic mediators of maternal immune tolerance to the developing feto-placental unit. Functional evaluation of T cells at the maternal-fetal interface is crucial to elucidate the immunologic basis of obstetric complications. Our objective was to define the T cell phenotype and function of uterine intervillous blood (IVB) in pregnancy with and without preeclampsia. We hypothesize that preeclampsia is associated with impaired immune tolerance and a pro-inflammatory uterine T cell microenvironment. In this cross-sectional study, maternal peripheral blood (PB) and uterine IVB (obtained from the surgical sponge used to clean the placental bed during cesarean delivery) were collected from participants with and without preeclampsia. Proportion, activation, and cytokine production of T cell subsets were quantified by flow cytometry. T cell parameters were compared by tissue source and by preeclampsia status. Sixty participants, 26 with preeclampsia, were included. Induced Treg made up a greater proportion of IVB T cells compared to PB and had greater cytokine-producing capacity. Preeclampsia was associated with increased ratio of pro-inflammatory IL-17α to suppressive IL-10 cytokine production by CD4 T cell subsets in IVB, but not in PB. Human uterine IVB is composed of activated, cytokine-producing T cell subsets distinct from maternal PB. Preeclampsia is associated with a pro-inflammatory IVB profile, with increased IL-17α /IL-10 ratio in all CD4 T cell subsets. IVB sampling is a useful tool for investigating human T cell biology at the maternal-fetal interface that may inform immunotherapeutic strategies for preeclampsia.

Waning Immunity Against XBB.1.5 Following Bivalent mRNA Boosters

The SARS-CoV-2 Omicron variant has continued to evolve. XBB is a recombinant between two BA.2 sublineages, XBB.1 includes the G252V mutation, and XBB.1.5 includes the G252V and F486P mutations. XBB.1.5 has rapidly increased in frequency and has become the dominant virus in New England. The bivalent mRNA vaccine boosters have been shown to increase neutralizing antibody (NAb) titers to multiple variants, but the durability of these responses remains to be determined. We assessed humoral and cellular immune responses in 30 participants who received the bivalent mRNA boosters and performed assays at baseline prior to boosting, at week 3 after boosting, and at month 3 after boosting. Our data demonstrate that XBB.1.5 substantially escapes NAb responses but not T cell responses after bivalent mRNA boosting. NAb titers to XBB.1 and XBB.1.5 were similar, suggesting that the F486P mutation confers greater transmissibility but not increased immune escape. By month 3, NAb titers to XBB.1 and XBB.1.5 declined essentially to baseline levels prior to boosting, while NAb titers to other variants declined less strikingly.

Durability of Heterologous and Homologous COVID-19 Vaccine Boosts

IMPORTANCE

Antibody responses elicited by current messenger RNA (mRNA) COVID-19 vaccines decline rapidly and require repeated boosting.

OBJECTIVE

To evaluate the immunogenicity and durability of heterologous and homologous prime-boost regimens involving the adenovirus vector vaccine Ad26.COV2.S and the mRNA vaccine BNT162b2.

DESIGN, SETTING, AND PARTICIPANTS

In this cohort study at a single clinical site in Boston, Massachusetts, 68 individuals who were vaccinated at least 6 months previously with 2 immunizations of BNT162b2 were boosted with either Ad26.COV2.S or BNT162b2. Enrollment of participants occurred from August 12, 2021, to October 25, 2021, and this study involved 4 months of follow-up. Data analysis was performed from November 2021 to February 2022.

EXPOSURES

Participants who were previously vaccinated with BNT162b2 received a boost with either Ad26.COV2.S or BNT162b2.

MAIN OUTCOMES AND MEASURES

Humoral immune responses were assessed by neutralizing, binding, and functional antibody responses for 16 weeks following the boost. CD8+ and CD4+ T-cell responses were evaluated by intracellular cytokine staining assays.

RESULTS

Among 68 participants who were originally vaccinated with BNT162b2 and boosted with Ad26.COV2.S (41 participants; median [range] age, 36 [23-84] years) or BNT162b2 (27 participants; median [range] age, 35 [23-76] years), 56 participants (82%) were female, 7 (10%) were Asian, 4 (6%) were Black, 4 (6%) were Hispanic or Latino, 3 (4%) were more than 1 race, and 53 (78%) were White. Both vaccines were found to be associated with increased humoral and cellular immune responses, including against SARS-CoV-2 variants of concern. BNT162b2 boosting was associated with a rapid increase of Omicron neutralizing antibodies that peaked at a median (IQR) titer of 1018 (699-1646) at week 2 and declined by 6.9-fold to a median (IQR) titer of 148 (95-266) by week 16. Ad26.COV2.S boosting was associated with increased Omicron neutralizing antibodies titers that peaked at a median (IQR) of 859 (467-1838) week 4 and declined by 2.1-fold to a median (IQR) of 403 (208-1130) by week 16.

CONCLUSIONS AND RELEVANCE

Heterologous Ad26.COV2.S boosting was associated with durable humoral and cellular immune responses in individuals who originally received the BNT162b2 vaccine. These data suggest potential benefits of heterologous prime-boost vaccine regimens for SARS-CoV-2.

Reduced SARS-CoV-2 disease outcomes in Syrian hamsters receiving immune sera: Quantitative image analysis in pathologic assessments

There is a need to standardize pathologic endpoints in animal models of SARS-CoV-2 infection to help benchmark study quality, improve cross-institutional comparison of data, and assess therapeutic efficacy so that potential drugs and vaccines for SARS-CoV-2 can rapidly advance. The Syrian hamster model is a tractable small animal model for COVID-19 that models clinical disease in humans. Using the hamster model, the authors used traditional pathologic assessment with quantitative image analysis to assess disease outcomes in hamsters administered polyclonal immune sera from previously challenged rhesus macaques. The authors then used quantitative image analysis to assess pathologic endpoints across studies performed at different institutions using different tissue processing protocols. The authors detail pathological features of SARS-CoV-2 infection longitudinally and use immunohistochemistry to quantify myeloid cells and T lymphocyte infiltrates during SARS-CoV-2 infection. High-dose immune sera protected hamsters from weight loss and diminished viral replication in tissues and reduced lung lesions. Cumulative pathology scoring correlated with weight loss and was robust in distinguishing IgG efficacy. In formalin-infused lungs, quantitative measurement of percent area affected also correlated with weight loss but was less robust in non-formalin-infused lungs. Longitudinal immunohistochemical assessment of interstitial macrophage infiltrates showed that peak infiltration corresponded to weight loss, yet quantitative assessment of macrophage, neutrophil, and CD3+ T lymphocyte numbers did not distinguish IgG treatment effects. Here, the authors show that quantitative image analysis was a useful adjunct tool for assessing SARS-CoV-2 treatment outcomes in the hamster model.

A homologous or variant booster vaccine after Ad26.COV2.S immunization enhances SARS-CoV-2-specific immune responses in rhesus macaques

Ad26.COV2.S has demonstrated durability and clinical efficacy against symptomatic COVID-19 in humans. In this study, we report the correlates of durability of humoral and cellular immune responses in 20 rhesus macaques immunized with single-shot Ad26.COV2.S and the immunogenicity of a booster shot at 8 to 10 months after the initial immunization. Ad26.COV2.S elicited durable binding and neutralizing antibodies as well as memory B cells and long-lived bone marrow plasma cells. Innate immune responses and bone marrow plasma cell responses correlated with durable antibody responses. After Ad26.COV2.S boost immunization, binding and neutralizing antibody responses against multiple SARS-CoV-2 variants increased 31- to 69-fold and 23- to 43-fold, respectively, compared with preboost concentrations. Antigen-specific B cell and T cell responses also increased substantially after the boost immunization. Boosting with a modified Ad26.COV2.S.351 vaccine expressing the SARS-CoV-2 spike protein from the beta variant led to largely comparable responses with slightly higher beta- and omicron-specific humoral immune responses. These data demonstrate that a late boost with Ad26.COV2.S or Ad26.COV2.S.351 resulted in a marked increase in humoral and cellular immune responses that were highly cross-reactive across multiple SARS-CoV-2 variants in rhesus macaques.

Durability and expansion of neutralizing antibody breadth following Ad26.COV2.S vaccination of mice

Emerging SARS-CoV-2 variants with the potential to escape binding and neutralizing antibody responses pose a threat to vaccine efficacy. We recently reported expansion of broadly neutralizing activity of vaccine-elicited antibodies in humans 8 months following a single immunization with Ad26.COV2.S. Here, we assessed the 15-month durability of antibody responses and their neutralizing capacity to B.1.617.2 (delta) and B.1.351 (beta) variants following a single immunization of Ad26.COV2.S in mice. We report the persistence of binding and neutralizing antibody titers following immunization with a concomitant increase in neutralizing antibody breadth to delta and beta variants over time. Evaluation of bone marrow and spleen at 15 months postimmunization revealed that Ad26.COV2.S-immunized mice tissues contained spike-specific antibody-secreting cells. We conclude that immunization with Ad26.COV2.S elicits a robust immune response against SARS-CoV-2 spike, which expands over time to neutralize delta and beta variants more robustly, and seeds bone marrow and spleen with long-lived spike-specific antibody-secreting cells. These data extend previous findings in humans and support the use of a mouse model as a potential tool to further explore the dynamics of the humoral immune response following vaccination with Ad26.COV2.S.

Immunity elicited by natural infection or Ad26.COV2.S vaccination protects hamsters against SARS-CoV-2 variants of concern

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern have emerged and may pose a threat to both the efficacy of vaccines based on the original WA1/2020 strain and the natural immunity induced by infection with earlier SARS-CoV-2 variants. We investigated how mutations in the spike protein of circulating SARS-CoV-2 variants, which have been shown to partially evade neutralizing antibodies, affect natural and vaccine-induced immunity. We adapted a Syrian hamster model of moderate to severe clinical disease for two variant strains of SARS-CoV-2: B.1.1.7 (alpha variant) and B.1.351 (beta variant). We then assessed the protective efficacy conferred by either natural immunity from WA1/2020 infection or by vaccination with a single dose of the adenovirus serotype 26 vaccine, Ad26.COV2.S. Primary infection with the WA1/2020 strain provided potent protection against weight loss and viral replication in lungs after rechallenge with WA1/2020, B.1.1.7, or B.1.351. Ad26.COV2.S induced cross-reactive binding and neutralizing antibodies that were reduced against the B.1.351 strain compared with WA1/2020 but nevertheless still provided robust protection against B.1.351 challenge, as measured by weight loss and pathology scoring in the lungs. Together, these data support hamsters as a preclinical model to study protection against emerging variants of SARS-CoV-2 conferred by prior infection or vaccination.

Protective efficacy of Ad26.COV2.S against SARS-CoV-2 B.1.351 in macaques

The emergence of SARS-CoV-2 variants that partially evade neutralizing antibodies poses a threat to the efficacy of current COVID-19 vaccines1,2. The Ad26.COV2.S vaccine expresses a stabilized spike protein from the WA1/2020 strain of SARS-CoV-2, and has recently demonstrated protective efficacy against symptomatic COVID-19 in humans in several geographical regions-including in South Africa, where 95% of sequenced viruses in cases of COVID-19 were the B.1.351 variant3. Here we show that Ad26.COV2.S elicits humoral and cellular immune responses that cross-react with the B.1.351 variant and protects against B.1.351 challenge in rhesus macaques. Ad26.COV2.S induced lower binding and neutralizing antibodies against B.1.351 as compared to WA1/2020, but elicited comparable CD8 and CD4 T cell responses against the WA1/2020, B.1.351, B.1.1.7, P.1 and CAL.20C variants. B.1.351 infection of control rhesus macaques resulted in higher levels of virus replication in bronchoalveolar lavage and nasal swabs than did WA1/2020 infection. Ad26.COV2.S provided robust protection against both WA1/2020 and B.1.351, although we observed higher levels of virus in vaccinated macaques after B.1.351 challenge. These data demonstrate that Ad26.COV2.S provided robust protection against B.1.351 challenge in rhesus macaques. Our findings have important implications for vaccine control of SARS-CoV-2 variants of concern.

Passive Transfer of Vaccine-Elicited Antibodies Protects against SIV in Rhesus Macaques

Several HIV-1 and SIV vaccine candidates have shown partial protection against viral challenges in rhesus macaques. However, the protective efficacy of vaccine-elicited polyclonal antibodies has not previously been demonstrated in adoptive transfer studies in nonhuman primates. In this study, we show that passive transfer of purified antibodies from vaccinated macaques can protect naive animals against SIVmac251 challenges. We vaccinated 30 rhesus macaques with Ad26-SIV Env/Gag/Pol and SIV Env gp140 protein vaccines and assessed the induction of antibody responses and a putative protective signature. This signature included multiple antibody functions and correlated with upregulation of interferon pathways in vaccinated animals. Adoptive transfer of purified immunoglobulin G (IgG) from the vaccinated animals with the most robust protective signatures provided partial protection against SIVmac251 challenges in naive recipient rhesus macaques. These data demonstrate the protective efficacy of purified vaccine-elicited antiviral antibodies in this model, even in the absence of virus neutralization.

Correlates of protection against SARS-CoV-2 in rhesus macaques

Recent studies have reported protective efficacy of both natural immunity1 and vaccine-induced immunity^2–7 against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) challenge in rhesus macaques. However, the importance of humoral and cellular immunity for protection against SARS-CoV-2 infection remains to be determined. Here we show that adoptive transfer of purified IgG from convalescent macaques protects naïve recipient rhesus macaques against SARS-CoV-2 challenge in a dose dependent fashion. Depletion of CD8+ T cells in convalescent animals partially abrogated the protective efficacy of natural immunity against SARS-CoV-2 re-challenge, suggesting the importance of cellular immunity in the context of waning or subprotective antibody titers. These data demonstrate that relatively low antibody titers are sufficient for protection against SARS-CoV-2 in rhesus macaques, and that cellular immune responses may also contribute to protection if antibody responses are suboptimal. We also show that higher antibody titers are required for therapy of SARS-CoV-2 infection in macaques. These findings have important implications for the development of SARS-CoV-2 vaccines and immune-based therapeutics.

Ad26 vaccine protects against SARS-CoV-2 severe clinical disease in hamsters

Coronavirus disease 2019 (COVID-19) in humans is often a clinically mild illness, but some individuals develop severe pneumonia, respiratory failure and death1-4. Studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in hamsters5-7 and nonhuman primates8-10 have generally reported mild clinical disease, and preclinical SARS-CoV-2 vaccine studies have demonstrated reduction of viral replication in the upper and lower respiratory tracts in nonhuman primates11-13. Here we show that high-dose intranasal SARS-CoV-2 infection in hamsters results in severe clinical disease, including high levels of virus replication in tissues, extensive pneumonia, weight loss and mortality in a subset of animals. A single immunization with an adenovirus serotype 26 vector-based vaccine expressing a stabilized SARS-CoV-2 spike protein elicited binding and neutralizing antibody responses and protected against SARS-CoV-2-induced weight loss, pneumonia and mortality. These data demonstrate vaccine protection against SARS-CoV-2 clinical disease. This model should prove useful for preclinical studies of SARS-CoV-2 vaccines, therapeutics and pathogenesis.

DNA vaccine protection against SARS-CoV-2 in rhesus macaques

The global coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has made the development of a vaccine a top biomedical priority. In this study, we developed a series of DNA vaccine candidates expressing different forms of the SARS-CoV-2 spike (S) protein and evaluated them in 35 rhesus macaques. Vaccinated animals developed humoral and cellular immune responses, including neutralizing antibody titers at levels comparable to those found in convalescent humans and macaques infected with SARS-CoV-2. After vaccination, all animals were challenged with SARS-CoV-2, and the vaccine encoding the full-length S protein resulted in >3.1 and >3.7 log₁₀ reductions in median viral loads in bronchoalveolar lavage and nasal mucosa, respectively, as compared with viral loads in sham controls. Vaccine-elicited neutralizing antibody titers correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate vaccine protection against SARS-CoV-2 in nonhuman primates.

Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques

A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic^1-8. For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in non-human primates. Fifty-two rhesus macaques (Macaca mulatta) were immunized with Ad26 vectors that encoded S variants or sham control, and then challenged with SARS-CoV-2 by the intranasal and intratracheal routes^9,10. The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs after SARS-CoV-2 challenge. Titres of vaccine-elicited neutralizing antibodies correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in non-human primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials.

SARS-CoV-2 infection protects against rechallenge in rhesus macaques

An understanding of protective immunity to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is critical for vaccine and public health strategies aimed at ending the global coronavirus disease 2019 (COVID-19) pandemic. A key unanswered question is whether infection with SARS-CoV-2 results in protective immunity against reexposure. We developed a rhesus macaque model of SARS-CoV-2 infection and observed that macaques had high viral loads in the upper and lower respiratory tract, humoral and cellular immune responses, and pathologic evidence of viral pneumonia. After the initial viral clearance, animals were rechallenged with SARS-CoV-2 and showed 5 log₁₀ reductions in median viral loads in bronchoalveolar lavage and nasal mucosa compared with after the primary infection. Anamnestic immune responses after rechallenge suggested that protection was mediated by immunologic control. These data show that SARS-CoV-2 infection induced protective immunity against reexposure in nonhuman primates.

Comparison of shortened mosaic HIV-1 vaccine schedules: a randomised, double-blind, placebo-controlled phase 1 trial (IPCAVD010/HPX1002) and a preclinical study in rhesus monkeys (NHP 17-22)

BACKGROUND

Current efficacy studies of a mosaic HIV-1 prophylactic vaccine require four vaccination visits over one year, which is a complex regimen that could prove challenging for vaccine delivery at the community level, both for recipients and clinics. In this study, we evaluated the safety, tolerability, and immunogenicity of shorter, simpler regimens of trivalent Ad26.Mos.HIV expressing mosaic HIV-1 Env/Gag/Pol antigens combined with aluminium phosphate-adjuvanted clade C gp140 protein.

METHODS

We did this randomised, double-blind, placebo-controlled phase 1 trial (IPCAVD010/HPX1002) at Beth Israel Deaconess Medical Center in Boston, MA, USA. We included healthy, HIV-uninfected participants (aged 18-50 years) who were considered at low risk for HIV infection and had not received any vaccines in the 14 days before study commencement. We randomly assigned participants via a computer-generated randomisation schedule and interactive web response system to one of three study groups (1:1:1) testing different regimens of trivalent Ad26.Mos.HIV (5 × 10¹⁰ viral particles per 0·5 mL) combined with 250 μg adjuvanted clade C gp140 protein. They were then assigned to treatment or placebo subgroups (5:1) within each of the three main groups. Participants and investigators were masked to treatment allocation until the end of the follow-up period. Group 1 received Ad26.Mos.HIV alone at weeks 0 and 12 and Ad26.Mos.HIV plus adjuvanted gp140 at weeks 24 and 48. Group 2 received Ad26.Mos.HIV plus adjuvanted gp140 at weeks 0, 12, and 24. Group 3 received Ad26.Mos.HIV alone at week 0 and Ad26.Mos.HIV plus adjuvanted gp140 at weeks 8 and 24. Participants in the control group received 0·5 mL of 0·9% saline. All study interventions were administered intramuscularly. The primary endpoints were Env-specific binding antibody responses at weeks 28, 52, and 72 and safety and tolerability of the vaccine regimens for 28 days after the injection. All participants who received at least one vaccine dose or placebo were included in the safety analysis; immunogenicity was analysed using the per-protocol population. The IPCAVD010/HPX1002 trial is registered with ClinicalTrials.gov, NCT02685020. We also did a parallel preclinical study in rhesus monkeys to test the protective efficacy of the shortened group 3 regimen.

FINDINGS

Between March 7, 2016, and Aug 19, 2016, we randomly assigned 36 participants to receive at least one dose of study vaccine or placebo, ten to each vaccine group and two to the corresponding placebo group. 30 (83%) participants completed the full study, and six (17%) discontinued it prematurely because of loss to follow-up, withdrawal of consent, investigator decision, and an unrelated death from a motor vehicle accident. The two shortened regimens elicited comparable antibody titres against autologous clade C Env at peak immunity to the longer, 12-month regimen: geometric mean titre (GMT) 41 007 (95% CI 17 959-93 636) for group 2 and 49 243 (29 346-82 630) for group 3 at week 28 compared with 44 590 (19 345-102 781) for group 1 at week 52). Antibody responses remained increased (GMT >5000) in groups 2 and 3 at week 52 but were highest in group 1 at week 72. Antibody-dependent cellular phagocytosis, Env-specific IgG3, tier 1A neutralising activity, and broad cellular immune responses were detected in all groups. All vaccine regimens were well tolerated. Mild-to-moderate pain or tenderness at the injection site was the most commonly reported solicited local adverse event, reported by 28 vaccine recipients (93%) and two placebo recipients (33%). Grade 3 solicited systemic adverse events were reported by eight (27%) vaccine recipients and no placebo recipients; the most commonly reported grade 3 systemic symptoms were fatigue, myalgia, and chills. The shortened group 3 regimen induced comparable peak immune responses in 30 rhesus monkeys as in humans and resulted in an 83% (95% CI 38·7-95, p=0·004 log-rank test) reduction in per-exposure acquisition risk after six intrarectal challenges with SHIV-SF162P3 at week 54, more than 6 months after final vaccination.

INTERPRETATION

Short, 6-month regimens of a mosaic HIV-1 prophylactic vaccine elicited robust HIV-specific immune responses that were similar to responses elicited by a longer, 12-month schedule. Preclinical data showed partial protective efficacy of one of the short vaccine regimens in rhesus monkeys. Further clinical studies are required to test the suitability of the shortened vaccine regimens in humans. Such shortened regimens would be valuable to increase vaccine delivery at the community level, particularly in resource-limited settings.

FUNDING

Ragon Institute (Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard University; Cambridge, MA, USA) and Janssen Vaccines & Prevention (Leiden, Netherlands).

Lack of therapeutic efficacy of an antibody to α4β7 in SIVmac251-infected rhesus macaques

Sustained virologic control of human immunodeficiency virus type 1 (HIV-1) infection after discontinuation of antiretroviral therapy (ART) is a major goal of the HIV-1 cure field. A recent study reported that administration of an antibody against α₄β₇ induced durable virologic control after ART discontinuation in 100% of rhesus macaques infected with an attenuated strain of simian immunodeficiency virus (SIV) containing a stop codon in nef We performed similar studies in 50 rhesus macaques infected with wild-type, pathogenic SIVmac251. In animals that initiated ART during either acute or chronic infection, anti-α₄β₇ antibody infusion had no detectable effect on the viral reservoir or viral rebound after ART discontinuation. These data demonstrate that anti-α₄β₇ antibody administration did not provide therapeutic efficacy in the model of pathogenic SIVmac251 infection of rhesus macaques.