Worldwide, ∼90% of all HIV infections occur through mucosal exposure and almost always involve CCR5 (R5)-tropic HIV strains that are relatively difficult to neutralize (tier 2). After acute infections, IgM is the first antibody class to respond. IgM is the only antibody present in all vertebrates . It is required for the maturation of IgG responses , regulation of B-cell development , modulating inflammatory responses , agglutination of pathogens, and clearance of apoptotic cells via complement activation . IgM exists as membrane-bound immunoglobulin on the surface of B cells, forming the B-cell receptor . Plasma IgM is mainly pentameric and contains the joining (J)-chain . At mucosal sites, IgM is produced locally by plasma cells in the lamina propria. After its production, IgM binds to the polymeric immunoglobulin receptor (pIgR) expressed on the basolateral surface of the epithelial barrier to form pIgR–IgM complexes. The latter are transported across the epithelial monolayer in transcytotic vesicles and released at the luminal side through proteolytic cleavage of pIgR. This process results in the release of secretory component that remains associated with IgM, thus generating secretory IgM (SIgM). The role of IgM in preventing HIV transmission is currently unknown.
Preclinical vaccine efficacy studies rely on nonhuman primate models, especially Indian-origin rhesus macaques. However, as the envelope of the simian immunodeficiency virus (SIV) is so divergent that antibodies against HIV do not recognize the SIV envelope, simian-human immunodeficiency viruses (SHIVs) have been constructed; these chimeras carry HIV env in a SIVmac239 backbone. The R5 SHIV/rhesus macaque model is used to assess the protective potential of recombinant human anti-HIV Env monoclonal antibodies (mAbs) by passive immunization.
Here, we sought to test whether recombinant monoclonal IgM given mucosally could prevent infection of rhesus macaques after intrarectal SHIV challenge. Two-thirds of 33C6-IgM-treated rhesus macaques were completely protected, and in those with breakthrough infection, autologous neutralizing antibodies appeared earlier compared with untreated controls. Our data reveal for the first time the protective potential of mucosal anti-HIV IgM.
Cell lines, reagents and virus
TZM-bl cells were obtained through the NIH AIDS Reagent Program, Division of AIDS, NIAID, from J.C. Kappes, X. Wu and Tranzyme Inc. SHIV-1157ip gp120 was prepared as described previously . SHIV-1157ipEL-p stock [grown in rhesus macaque peripheral blood mononuclear cells (PBMC)] had a p27 concentration of 792 ng/ml and 7.8 × 105 50% tissue culture infectious doses (TCID50)/ml (measured in TZM-bl cells).
Preparation of 33C6 monoclonal antibodies and in-vitro assays, including surface plasmon resonance and dynamic light scattering
We previously described the production of 33C6-IgG1 mAb ; 33C6-IgM mAb was prepared and tested by ELISA, neutralization, avidity, surface plasmon resonance (SPR), dynamic light scattering (DLS) and virion capture assays as described in the Supplemental Digital Content, https://links.lww.com/QAD/B289.
Passive immunization and mucosal SHIV-1157ipEL-p challenge
All primate studies were conducted in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the USA (see Supplemental Digital Content, https://links.lww.com/QAD/B289). Rhesus macaques were randomized into groups (n = 6 per group). Groups 1 and 2 were given intrarectal mAbs at a total dose of 1.25 mg in 2.1 ml of PBS. Group 1 rhesus macaques received 33C6-IgM (1.26 nmol); Group 2 received 33C6-IgG1 (8.45 nmol). Group 3 (controls) received 2.1 ml of PBS intrarectal only.
Thirty minutes after mAb or PBS administration, rhesus macaques were atraumatically challenged intrarectally with 31.5 50% animal infectious doses (AID50) of the R5 clade C SHIV-1157ipEL-p . Plasma samples for mAb detection and viral load determination were obtained on the day of SHIV challenge and prospectively thereafter. Plasma viral RNA levels were measured as described previously .
Statistical analyses were performed using GraphPad Prism, version 7 for Windows (GraphPad Software Inc., La Jolla, California, USA). The time-to-peak viremia was analyzed by Kaplan–Meier analysis using the log-rank test with Holm–Sidak adjusted two-sided P values.
Generation of class-switched monoclonal antibody 33C6-IgM
We previously identified and produced mAb 33C6-IgG1 . To class-switch the latter to IgM, we cloned the heavy and light variable gene fragments in-frame with the human μ and λ chain constant regions, respectively. To express 33C6-IgM, we cotransfected the resulting vector constructs with the human J chain precursor expression plasmid  into Expi293 cells. We purified 33C6-IgM from filtered culture supernatant with thiophilic resin affinity binding and cation exchange chromatography. The presence of polymeric 33C6-IgM was confirmed with denaturing, nonreducing SDS-PAGE and western blot analysis (Fig. 1a) and by DLS (Fig. 1b, and Supplemental Digital Content, https://links.lww.com/QAD/B289).
33C6-IgM recognized and bound to the conserved V3 loop crown of HIV Env as expected based upon the known epitope specificity of 33C6-IgG1  (Fig. 1c). 33C6-IgM bound with greater avidity to SHIV-1157ip gp120 than 33C6-IgG1 (Fig. 1d), confirming the known superior avidity of IgM. SPR analysis revealed that 33C6-IgM bound to SHIV-1157ip gp120 with a KD of 2.5 pM (Fig. 1e). In contrast, 33C6-IgG1 had a KD of 130 pM (Fig. 1f), indicating that the IgM class bound significantly tighter than the IgG1 isotype of mAb 33C6. The on-rate for 33C6-IgM was 18-fold faster than that of the IgG1 version, and the IgM off-rate was 2.8 times slower than that of 33C6-IgG1. Together, these parameters account for the much tighter binding of the IgM isoform to soluble gp120 of the challenge virus.
33C6-IgM neutralized and captured SHIV in vitro
To assess the potential of 33C6-IgM to protect rhesus macaques against mucosal SHIV challenge – we tested the neutralization of SHIV-1157ipEL-p, the intended challenge virus strain, by 33C6-IgM and the IgG1 isotype by TZM-bl assay. This R5 clade C tier 1 SHIV strain had been used to demonstrate complete cross-clade protection of rhesus macaques by the human mAb HGN194 ; the latter has a similar epitope specificity as the 33C6 mAbs. 33C6-IgM neutralized SHIV-1157ipEL-p 10x better compared with 33C6-IgG1 (Fig. 2a). Of note, the pentameric IgM contains 5× more antigen-binding sites compared with IgG1 at the same molar concentration, which might explain the greater neutralization potency of 33C6-IgM.
Next, we performed virion capture assays with the two 33C6 mAbs as we had shown previously that virion capture correlated with protection against mucosal SHIV-1157ipEL-p challenge , the virus strain that was used in the current study. The IgM form depleted physical virus particles by 75% and infectious virions by 96% (Fig. 2b); compared with 33C6-IgG1, the IgM form depleted more physical particles. Importantly, both isoforms removed almost all infectious virions.
33C6-IgM protected rhesus macaques against high-dose mucosal SHIV challenge
Rhesus macaques were passively immunized intrarectally with 33C6-IgM (Group 1) or 33C6-IgG1 (Group 2); Group 3 (control) rhesus macaques were given intrarectal PBS only (Fig. 3). After 30 min, all rhesus macaques were challenged intrarectally with 31.5 50% animal infectious doses (AID50) of SHIV-1157ipEL-p, and plasma viral RNA (vRNA) levels were monitored for 12 weeks. Four out of six (67%) rhesus macaques in Group 1 (33C6-IgM; Fig. 4a), and five out of six (83%) rhesus macaques in Group 2 (33C6-IgG1; Fig. 4b) remained aviremic. In contrast, all control rhesus macaques became systemically infected by week 2 with a median peak viremia of 106 vRNA copies/ml (Fig. 4c); the three rhesus macaques with breakthrough infection in Groups 1 and 2 had peak vRNA levels greater than 106 copies/ml. The time-to-peak viremia was analyzed by Kaplan–Meier analysis using the log-rank test with Holm–Sidak adjusted two-sided P-values (Fig. 4d). Passively immunized rhesus macaques in Groups 1 and 2 had delayed peak viremia compared with Group 3 controls (Group 1, P = 0.009; and Group 2, P = 0.043). Of note, the 1.25 mg mAb dose used in this study contains five-fold fewer IgM than IgG1 molecules. Thus, at equimolar concentrations, IgM might be better than IgG1. Taken together, data demonstrate that IgM is as effective as IgG1 in protecting against mucosal SHIV acquisition.
33C6-IgM treatment accelerated the induction of anti-SHIV neutralizing antibodies
Next, we sought to examine the development of SHIV-specific antibodies in rhesus macaques with breakthrough infection. All infected rhesus macaques seroconverted (Fig. 5a); in passively immunized rhesus macaques of Groups 1 and 2 that became infected, anti-Env antibodies became detectable with delays.
We then tested plasma samples from all rhesus macaques with systemic SHIV infection for neutralizing antibodies against the SHIV-1157ipEL-p challenge virus (Fig. 5b and c). Among day 42 plasma samples, neutralizing antibodies were only seen in one rhesus macaque animal 33011 treated with 33C6-IgM (Fig. 5b). By day 84, plasma from both of the infected, IgM-treated rhesus macaques neutralized the challenge SHIV by at least 50% (Fig. 5c). In contrast, neutralizing antibodies were present in only one out of six controls, indicating a trend for earlier development of autologous neutralizing antibodies in the IgM-treated rhesus macaques with breakthrough infection (P = 0.1). We conclude that mucosally administered IgM protects against high-dose mucosal SHIV challenge and may accelerate the development of autologous neutralizing antibodies in case virus acquisition is not prevented.
Here, we give the first report of preventing SHIV infection with a mucosally administered, anti-HIV 33C6-IgM. The majority of the passively immunized rhesus macaques was completely protected. In IgM-treated rhesus macaques with breakthrough infection, there was a trend for earlier development of autologous neutralizing antibodies than in virus-only controls. 33C6-IgM neutralized SHIV, captured physical virus particles and depleted infectious virions in vitro, suggesting potential protective mechanisms. Our data give proof-of-concept that IgM can be an effective first-line of defense at mucosal barrier.
33C6-IgM targets a protruding element of HIV Env, the conserved V3 loop crown that is easily accessible in the challenge virus. Consequently, 33C6-IgM potently neutralized the challenge SHIV and not only captured most of the physical virions present in the stock, but also removed close to 100% of the infectious particles. The IgG1 isotype was equally effective in eliminating infectious virions by capture. These characteristics can be explained by the binding profiles for 33C6-IgM and 33C6-IgG1 as assessed by avidity indices and SPR analyses; by both measures, 33C6-IgM showed tighter binding. The affinity of 33C6-IgM for the soluble challenge virus gp120 was extraordinarily high with a KD in the low picomolar range indicating that binding was 52-fold tighter than binding of the IgG1 isoform. A large part of this difference in affinity is also likely because of avidity effects, which may be even greater for binding to virions, where the presence of multiple copies of gp120 would allow a larger number of simultaneous interactions with the multiple binding sites on the IgM mAb. Such powerful avidity effects may contribute substantially to the greater neutralization ability of 33C6-IgM compared with the IgG1 isoform.
The dual action – direct neutralization and efficient infectious virion capture – is likely the underlying basis for the protection we observed in vivo for both mAb forms of 33C6; the difference in the degree of protection between 33C6-IgM and its IgG1 counterpart was not significant. We interpret our data that pentameric IgM can efficiently trap incoming virus by crosslinking and prevent mucosal transmission through immune exclusion.
The rules for HIV/SHIV immune exclusion in the mucosal compartment remain to be determined. We have demonstrated that intra-luminal administration of mAbs of different immunoglobulin classes prevents SHIV transmission; this includes recombinant monoclonal IgM, IgG, and dimeric IgA (dIgA) [8,13–15]. Other groups have focused on vaginal administration of anti-HIV IgG1 neutralizing mAbs [16,17]. The passive mucosal immunizations with human dIgA1 and dIgA2 isotypes involved mAbs with almost identical epitope specificity as that of 33C6-IgM, namely HGN194 . Both series of mAbs target the conserved gp120 V3 loop crown in R5-tropic HIV strains. HGN194 was isolated as an IgG1 from an infected person harboring an HIV clade AG circulating recombinant form. In contrast, 33C6-IgG1 was initially cloned from a single memory B cell from a rhesus macaque with chronic clade C SHIV infection; mAb 33C6-IgG1 specifically recognized a conformational mimotope representing the V3 loop . An unanswered question is whether mAbs targeting recessed epitopes can be effective for immune exclusion. Likewise, it remains to be determined whether mAbs need to be neutralizing in order to capture infectious virions.
Anti-HIV neutralizing mAbs have been class-switched from IgG to IgM, such as the IgG1 mAbs 2F5, 4E10, and 2G12. MAbs 2F5 and 4E10 target epitopes in the membrane proximal external region (MPER) of HIV gp41 known to be difficult to access . Class switching from IgG1 to IgM resulted in loss of neutralizing activity [19–21]. In contrast, the IgM isoform of 2G12, a mannose-dependent neutralizing Ab with epitopes located on the glycan shield , not only retained the ability to neutralize HIV but also actually neutralized the virus up to 28-fold more efficiently in PBMC cultures than the corresponding IgG1 isoform . The contrasting results obtained with anti-MPER and antiglycan mAbs can be explained by differential epitope accessibility. The 33C6 mAbs used in our study target the readily accessible, conserved V3 loop crown epitope of gp120 . This translated into efficient neutralization and virion capture by IgM and protection of rhesus macaques against a mucosal SHIV challenge.
Tomaras et al.[23,24] have examined specific antiviral IgM responses arising during acute HIV infection. IgM is the first antibody class to respond to infection or immunization. Thus, IgM responses have been used to diagnose new infections by various pathogens. Indeed, the very first free plasma antibodies detected in acute HIV infection involved IgM; these antibodies had autologous anti-gp41 specificity [23,24]. They appeared as early as 5 days after plasma viremia became detectable and were also found as virion–IgM complexes. Mathematical modeling showed that the early anti-gp41 IgM responses did not affect plasma viremia and thus were not felt to benefit the host . This important study focused on virus-specific IgM responses after the HIV transmission had already occurred. Here, we give evidence that passive immunoprophylaxis with anti-HIV IgM before virus exposure can be protective.
Passive immunization is considered a classical tool in immunology to determine the causal relationship between antibodies and protection against infectious agents. This includes mAbs with well characterized epitopes; as such, passive immunization data can serve as blue prints for immunogen selection and design. In general, induction of protective IgM responses has not been a defined goal for vaccine development because of the waning of early IgM responses that are replaced by IgG, IgA and other immunoglobulin class responses that arise by programmed class switching. Thus, IgM is not usually considered to play an important role in long-term immunity. Surprisingly, recent mouse model studies showed that IgM responses can be long-lived and contribute to long-term protection [25,26]. The significance of these findings for vaccine development remains to be determined. However, Dorfmeier et al. performed post-exposure vaccination and pathogenic RABV challenge studies within a short time window of 10 days. Their data pointed to the rapid induction of specific IgM responses as the key determinant of the vaccine-linked protection against lethal RABV challenge in the mice. In our passive immunization study with mAb 33C6-IgM, we also made the intriguing observation of unusually early appearance of SHIV-neutralizing antibody responses in the context of breakthrough infection despite passive IgM immunization. The underlying mechanism(s) responsible for the accelerated induction of virus-neutralizing antibody responses remain to be determined.
In conclusion, we have demonstrated that recombinant, monoclonal IgM protects against mucosal SHIV acquisition. Our data have implications for vaccine development in general and anti-HIV/AIDS vaccines in particular.
We thank A. Nabbale and J. Esquivel for assistance with the preparation of this manuscript, A.M. Sholukh for discussions, S.-L. Hu for SHIV-1157ip gp120, W. Marasco for mAb Fm-6-IgG1, and J. Mascola for mAb VRC01 obtained through the NIH AIDS Reagent Program, Division of AIDS, NIAID, NIH.
R.M.R. conceived and designed the project. S.G., K.T., V.K., S.K.L. designed and performed experiments and analyzed data. K.T., V.K., and S.K.L. coordinated the primate studies. L.W. and E.M.L. designed, performed, and analyzed the SPR and DLS experiments. O.S.A., D.H., A.S., and E.P. assisted with experiments and data analysis. S.J.R. performed the biostatistical analyses. S.G., K.T., and R.M.R. wrote the manuscript; all coauthors contributed and gave input.
Conflicts of interest
There are no conflicts of interest.
This project was supported by National Institutes of Health grants R01 AI100703 and P01 AI048240 to R.M.R. This study used resources supported by the Southwest National Primate Research Center (SNPRC) grant P51 OD011133 from the Office of Research Infrastructure Programs, National Institutes of Health. The SPR studies were performed in the University of Texas Health Science Center at San Antonio (UTHSCSA) Center for Macromolecular Interactions, which is supported by the Cancer Therapy and Research Center through the National Cancer Institute P30 Grant CA054174, and Texas State funds provided through the UTHSCSA Office of the Vice President for Research.
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* Siqi Gong and Khamis Tomusange contributed equally to this work.