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An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The pharmacokinetic properties of antibodies are largely dictated by the pH-dependent binding of the IgG fragment crystallizable (Fc) domain to the human neonatal Fc receptor (hFcRn). Engineered Fc domains that confer a longer circulation half-life by virtue of more favorable pH-dependent binding to hFcRn are of great therapeutic interest. Here we developed a pH Toggle switch Fc variant containing the L309D/Q311H/N434S (DHS) substitutions, which exhibits markedly improved pharmacokinetics relative to both native IgG1 and widely used half-life extension variants, both in conventional hFcRn transgenic mice and in new knock-in mouse strains. engineered specifically to recapitulate all the key processes relevant to human antibody persistence in circulation, namely: (i) physiological expression of hFcRn, (ii) the impact of hFcγRs on antibody clearance and (iii) the role of competing endogenous IgG. DHS-IgG retains intact effector functions, which are important for the clearance of target pathogenic cells and also has favorable developability.

Neuromuscular blocking agents (NMBAs) relax skeletal muscles to facilitate surgeries and ease intubation but can lead to adverse reactions, including complications because of postoperative residual neuromuscular blockade (rNMB) and, in rare cases, anaphylaxis. Both adverse reactions vary between types of NMBAs, with rocuronium, a widely used nondepolarizing NMBA, inducing one of the longest rNMB durations and highest anaphylaxis incidences. rNMB induced by rocuronium can be reversed by the synthetic γ-cyclodextrin sugammadex. However, in rare cases, sugammadex can provoke anaphylaxis. Thus, additional therapeutic options are needed. Rocuronium-induced anaphylaxis is proposed to rely on preexisting rocuronium-binding antibodies. To understand the pathogenesis of rocuronium-induced anaphylaxis and to identify potential therapeutics, we investigated the memory B cell antibody repertoire of patients with suspected hypersensitivity to rocuronium. We identified polyclonal antibody repertoires with a high diversity among V(D)J genes without evidence of clonal groups. When recombinantly expressed, these antibodies demonstrated specificity and low affinity for rocuronium without cross-reactivity for other NMBAs. Moreover, when these antibodies were expressed as human immunoglobulin E (IgE), they triggered human mast cell activation and passive systemic anaphylaxis in transgenic mice, although their affinities were insufficient to serve as reversal agents. Rocuronium-specific, high-affinity antibodies were thus isolated from rocuronium-immunized mice. The highest-affinity antibody was able to reverse rocuronium-induced neuromuscular blockade in nonhuman primates with kinetics comparable to that of sugammadex. Together, these data support the hypothesis that antibodies cause anaphylactic reactions to rocuronium and pave the way for improved diagnostics and neuromuscular blockade reversal agents. Editor's summary: Neuromuscular blocking agents (NMBAs) such as rocuronium are used during anesthesia to assist with surgeries and intubations. Although these agents are effective, they are not without risk. Patients can develop postoperative residual neuromuscular blockade, which can be treated with an NMBA reversal agent, and life-threatening anaphylaxis can occur in rare cases. Here, Dejoux et al. isolated and characterized memory B cells from three individuals who developed reactions to NMBA treatment and had serum antibodies to rocuronium. The authors found that IgE antibodies generated from these memory B cells were sufficient to induce anaphylaxis in mice treated with rocuronium, although they were too low affinity to be used as reversal agents. This led the authors to immunize mice with rocuronium to generate antibodies of higher affinity, which were able to reverse neuromuscular blockade in rocuronium-treated nonhuman primates. Together, these data explain why these three patients had a reaction and offer a therapeutic approach to reversing neuromuscular blockade. —Courtney Malo [ABSTRACT FROM AUTHOR]

Potent platelets: Anaphylaxis results from inappropriate immune responses to allergens. Human platelets express the IgG receptor FcγRIIA/CD32A and release inflammatory mediators in response to their engagement, but their contribution to anaphylaxis is not well understood. Beutier et al. developed mouse models that express either human FcγRIIA/CD32A alone or the full human IgG receptor complexity to understand the role of platelets in anaphylaxis. Anaphylaxis induced a marked decrease in platelet levels, but preventive platelet depletion reduced anaphylaxis severity. A clinical study of patients with drug-induced anaphylaxis revealed that a severe reaction was likewise associated with fewer circulating platelets. Activated platelets released serotonin, which contributed to anaphylaxis severity. These results reveal a critical role for platelets in IgG-mediated anaphylaxis. Platelets are key regulators of vascular integrity; however, their role in anaphylaxis, a life-threatening systemic allergic reaction characterized by the loss of vascular integrity and vascular leakage, remains unknown. Anaphylaxis is a consequence of inappropriate cellular responses triggered by antibodies to generally harmless antigens, resulting in a massive mediator release and rapidly occurring organ dysfunction. Human platelets express receptors for immunoglobulin G (IgG) antibodies and can release potent mediators, yet their contribution to anaphylaxis has not been previously addressed in mouse models, probably because mice do not express IgG receptors on platelets. We investigated the contribution of platelets to IgG-dependent anaphylaxis in human IgG receptor–expressing mouse models and a cohort of patients suffering from drug-induced anaphylaxis. Platelet counts dropped immediately and markedly upon anaphylaxis induction only when they expressed the human IgG receptor FcγRIIA/CD32A. Platelet depletion attenuated anaphylaxis, whereas thrombocythemia substantially worsened its severity. FcγRIIA-expressing platelets were directly activated by IgG immune complexes in vivo and were sufficient to restore susceptibility to anaphylaxis in resistant mice. Serotonin released by activated platelets contributed to anaphylaxis severity. Data from a cohort of patients suffering from drug-induced anaphylaxis indicated that platelet activation was associated with anaphylaxis severity and was accompanied by a reduction in circulating platelet numbers. Our findings identify platelets as critical players in IgG-dependent anaphylaxis and provide a rationale for the design of platelet-targeting strategies to attenuate the severity of anaphylactic reactions. [ABSTRACT FROM AUTHOR]

Hepatitis B virus (HBV) persistence may be due to impaired HBV-specific immune responses being unable to eliminate efficiently or cure infected hepatocytes. The immune mechanisms that lead to HBV persistence have not been completely identified, and no appropriate animal model is available for such studies. Therefore, we established a chronic HBV infection model in a mouse strain with human leukocyte antigen A2/DR1 (HLA-A2/DR1) transgenes and an H-2 class I/class II knockout. The liver of these mice was transduced with adeno-associated virus serotype 2/8 (AAV2/8) carrying a replication-competent HBV DNA genome. In all AAV2/8-transduced mice, hepatitis B virus surface antigen, hepatitis B virus e antigen, and HBV DNA persisted in serum for at least 1 year. Viral replication intermediates and transcripts were detected in the livers of the AAV-injected mice. The hepatitis B core antigen was expressed in 60% of hepatocytes. No significant inflammation was observed in the liver. This was linked to a higher number of regulatory T cells in liver than in controls and a defect in HBV-specific functional T-cell responses. Despite the substantial tolerance resulting from expression of HBV antigens in hepatocytes, we succeeded in priming functional HBV-specific T-cell responses in peripheral tissues, which subsequently reached the liver. This AAV2/8-HBV-transduced HLA-A2/DR1 murine model recapitulates virological and immunological characteristics of chronic HBV infection, and it could be useful for the development of new treatments and immune-based therapies or therapeutic vaccines for chronic HBV infections. [ABSTRACT FROM AUTHOR]

Rift Valley fever virus (RVFV) leads to varied clinical manifestations in animals and in humans that range from moderate fever to fatal illness, suggesting that host immune responses are important determinants of the disease severity. We investigated the immune basis for the extreme susceptibility of MBT/Pas mice that die with mild to acute hepatitis by day 3 post-infection compared to more resistant BALB/cByJ mice that survive up to a week longer. Lower levels of neutrophils observed in the bone marrow and blood of infected MBT/Pas mice are unlikely to be causative of increased RVFV susceptibility as constitutive neutropenia in specific mutant mice did not change survival outcome. However, whereas MBT/Pas mice mounted an earlier inflammatory response accompanied by higher amounts of interferon (IFN)-α in the serum compared to BALB/cByJ mice, they failed to prevent high viral antigen load. Several immunological alterations were uncovered in infected MBT/Pas mice compared to BALB/cByJ mice, including low levels of leukocytes that expressed type I IFN receptor subunit 1 (IFNAR1) in the blood, spleen and liver, delayed leukocyte activation and decreased percentage of IFN-γ-producing leukocytes in the blood. These observations are consistent with the complex mode of inheritance of RVFV susceptibility in genetic studies.