Association of the US President's Emergency Plan for AIDS Relief's Funding With Prevention of Mother-to-Child Transmission of HIV in Kenya.

Importance: From 2004 to 2014, the US President's Emergency Plan for AIDS Relief (PEPFAR) invested more than $248 000 000 in the prevention of mother-to-child transmission (PMTCT) of HIV in Kenya. Concurrently, child mortality in Kenya decreased by half. Objective: To identify the extent to which the decrease in child mortality in Kenya is associated with PEPFAR funding for PMTCT of HIV. Design, Setting, and Participants: This population-based survey study conducted in Kenya estimated the association between annual per capita PEPFAR funding for PMTCT (annual PCF) and cumulative per capita PEPFAR funding for PMTCT (cumulative PCF), extracted using 2004-2014 country operational reports as well as individual-level health outcomes, extracted from the 2003, 2008-2009, and 2014 Kenya Demographic and Health Surveys and the 2007 and 2012 Kenya AIDS Indicator Surveys. The study included children of female respondents to the 2003, 2008-2009, and 2014 Kenya Demographic and Health Surveys who were born 1 to 60 months (for neonatal mortality) or 12 to 60 months (for infant mortality) before the survey, as well as female respondents who had recently given birth and reported on HIV testing during antenatal care (ANC) during the 2007-2014 surveys. Results were adjusted for year, province, and survey respondent characteristics. Statistical analysis was performed from July 8, 2016, to December 10, 2018. Main Outcomes and Measures: Neonatal mortality was defined as death within the first month of life and infant mortality was defined as death within the first year of life. HIV testing during ANC was defined as receiving counseling on PMTCT, undergoing an HIV test, and receiving test results during ANC. Results: The analysis included 33 181 neonates (16 870 boys), 26 876 infants (13 679 boys), and 20 775 mothers (mean [SD] age, 28.0 [6.7] years). PEPFAR funding was not associated with neonatal mortality. A $0.33 increase in annual PCF, corresponding to the difference between the 75th and 25th (interquartile range) percentiles of funding, was significantly associated with a 16% (95% CI, 4%-27%) reduction in infant mortality after a 1-year lag. A 14% to 16% reduction persisted after 2- and 3-year lags, and comparable reductions were observed for unlagged and 1-year lagged cumulative PCF. An increase of 1 interquartile range in cumulative PCF was associated with a 7% (95% CI, 3%-11%) increase in HIV testing during ANC, which intensified with subsequent lags. Between 2004 and 2014, sustained funding levels of $0.33 annual PCF could have averted 118 039 to 273 924 infant deaths. Conclusions and Relevance: Evidence from publicly available data suggests that PEPFAR's PMTCT funding was associated with a reduction in infant mortality and an increase in HIV testing during ANC in Kenya. The full outcome of funding may not be realized until several years after allocation.

Human IgG Fc domain engineering enhances antitoxin neutralizing antibody activity.

The effector activity of antibodies is dependent on engagement with Fcγ receptors (FcγRs) and activation of the associated intracellular signaling pathways. Preclinical evaluation of therapeutic humanized or chimeric mAbs to study the interactions of their Fc regions with FcγRs is hampered by substantial structural and functional FcγR diversity among species. In this report, we used mice expressing only human FcγRs to evaluate the contribution of FcγR-mediated pathways to the neutralizing activity of an anti-anthrax toxin chimeric mAb. We observed that the protective activity of this mAb was highly dependent upon FcγR engagement, with minimal protection against anthrax toxin observed in FcγR-deficient mice following mAb administration. We generated anti-anthrax toxin mAbs with specific Fc domain variants with selectively enhanced affinity for particular human FcγRs and assessed their activity in FcγR-humanized mice. We determined that Fc domain variants that were capable of selectively engaging activating FcγRs substantially enhanced the in vitro and in vivo activity of anthrax toxin-neutralizing antibodies. These findings indicate that the application of Fc domain engineering is a feasible strategy to enhance toxin-neutralizing activity and suggest that engineered antitoxin antibodies will have improved therapeutic efficacy.

A requirement for FcγR in antibody-mediated bacterial toxin neutralization.

One important function of humoral immunity is toxin neutralization. The current view posits that neutralization results from antibody-mediated interference with the binding of toxins to their targets, a phenomenon viewed as dependent only on antibody specificity. To investigate the role of antibody constant region function in toxin neutralization, we generated IgG2a and IgG2b variants of the Bacillus anthracis protective antigen-binding IgG1 monoclonal antibody (mAb) 19D9. These antibodies express identical variable regions and display the same specificity. The efficacy of antibody-mediated neutralization was IgG2a > IgG2b > IgG1, and neutralization activity required competent Fcγ receptor (FcγR). The IgG2a mAb prevented lethal toxin cell killing and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase cleavage more efficiently than the IgG1 mAb. Passive immunization with IgG1 and IgG2a mAb protected wild-type mice, but not FcγR-deficient mice, against B. anthracis infection. These results establish that constant region isotype influences toxin neutralization efficacy of certain antibodies through a mechanism that requires engagement of FcγR. These findings highlight a new parameter for evaluating vaccine responses and the possibility of harnessing optimal FcγR interactions in the design of passive immunization strategies.

Identification of linear epitopes in Bacillus anthracis protective antigen bound by neutralizing antibodies.

Protective antigen (PA), the binding subunit of anthrax toxin, is the major component in the current anthrax vaccine, but the fine antigenic structure of PA is not well defined. To identify linear neutralizing epitopes of PA, 145 overlapping peptides covering the entire sequence of the protein were synthesized. Six monoclonal antibodies (mAbs) and antisera from mice specific for PA were tested for their reactivity to the peptides by enzyme-linked immunosorbent assays. Three major linear immunodominant B-cell epitopes were mapped to residues Leu(156) to Ser(170), Val(196) to Ile(210), and Ser(312) to Asn(326) of the PA protein. Two mAbs with toxin-neutralizing activity recognized two different epitopes in close proximity to the furin cleavage site in domain 1. The three-dimensional complex structure of PA and its neutralizing mAbs 7.5G and 19D9 were modeled using the molecular docking method providing models for the interacting epitope and paratope residues. For both mAbs, LeTx neutralization was associated with interference with furin cleavage, but they differed in effectiveness depending on whether they bound on the N- or C-terminal aspect of the cleaved products. The two peptides containing these epitopes that include amino acids Leu(156)-Ser(170) and Val(196)-Ile(210) were immunogenic and elicited neutralizing antibody responses to PA. These results identify the first linear neutralizing epitopes of PA and show that peptides containing epitope sequences can elicit neutralizing antibody responses, a finding that could be exploited for vaccine design.

Immunogenicity of Bacillus anthracis protective antigen domains and efficacy of elicited antibody responses depend on host genetic background.

Neutralizing antibodies to Bacillus anthracis protective antigen (PA), a component of anthrax toxin, mediate protection against anthrax. PA is antigenically complex and can elicit protective and nonprotective antibodies. Furthermore, vaccinated individuals demonstrate considerable variability in their antibody responses to PA. To explore the relationship between PA structure and antigenicity, we produced Escherichia coli strains expressing full-length PA (PA1-4), domains 2 to 4 (PA2-4), domain 1, (PA1), and domain 4 (PA4) and evaluated the immunogenicities and protective efficacies of the protein fractions in four mouse strains (strains A/J, BALB/c, C57BL/6, and Swiss Webster). Immunization with PA1-4 resulted in significantly higher lethal toxin-neutralizing antibody titers than immunization with any recombinant protein (rPA) fraction of PA. The magnitude and neutralizing capacity of the antibody response to full-length PA and its fragments varied depending on the mouse strain. We found no correlation between the antibody titer and the neutralizing antibody titer for A/J and Swiss Webster mice. In C57BL/6 mice, antibody titers and neutralization capacity correlated for two of four rPA domain proteins tested, while BALB/c mice displayed a similar correlation with only one rPA. By correlating the reactivity of immune sera with solvent-exposed linear peptide segments of PA, we tentatively assign the presence of four new linear B-cell epitopes in PA amino acids 121 to 150, 143 to 158, 339 to 359, and 421 to 440. We conclude that the genetic background of the host determines the relative efficacy of the antitoxin response. The results suggest that the variability observed in vaccination studies with PA-derived vaccines is a result of host heterogeneity and implies a need to develop other antigens as vaccine candidates.

A monoclonal antibody to Bacillus anthracis protective antigen defines a neutralizing epitope in domain 1.

Antibody (Ab) responses to Bacillus anthracis toxins are protective, but relatively few protective monoclonal antibodies (MAbs) have been reported. Protective antigen (PA) is essential for the action of B. anthracis lethal toxin (LeTx) and edema toxin. In this study, we generated two MAbs to PA, MAbs 7.5G and 10F4. These MAbs did not compete for binding to PA, consistent with specificities for different epitopes. The MAbs were tested for their ability to protect a monolayer of cultured macrophages against toxin-mediated cytotoxicity. MAb 7.5G, the most-neutralizing MAb, bound to domain 1 of PA and reduced LeTx toxicity in BALB/c mice. Remarkably, MAb 7.5G provided protection without blocking the binding of PA or lethal factor or the formation of the PA heptamer complex. However, MAb 7.5G slowed the proteolytic digestion of PA by furin in vitro, suggesting a potential mechanism for Ab-mediated protection. These observations indicate that some Abs to domain 1 can contribute to host protection.