Approximately 5.1 million Israelis had been fully immunized against coronavirus disease 2019 (Covid-19) after receiving two doses of the BNT162b2 messenger RNA vaccine (Pfizer–BioNTech) by May 31, 2021. After early reports of myocarditis during adverse events monitoring, the Israeli Ministry of Health initiated active surveillance. We retrospectively reviewed data obtained from December 20, 2020, to May 31, 2021, regarding all cases of myocarditis and categorized the information using the Brighton Collaboration definition. We analyzed the occurrence of myocarditis by computing the risk difference for the comparison of the incidence after the first and second vaccine doses (21 days apart); by calculating the standardized incidence ratio of the observed-to-expected incidence within 21 days after the first dose and 30 days after the second dose, independent of certainty of diagnosis; and by calculating the rate ratio 30 days after the second dose as compared with unvaccinated persons. Among 304 persons with symptoms of myocarditis, 21 had received an alternative diagnosis. Of the remaining 283 cases, 142 occurred after receipt of the BNT162b2 vaccine; of these cases, 136 diagnoses were definitive or probable. The clinical presentation was judged to be mild in 129 recipients (95%); one fulminant case was fatal. The overall risk difference between the first and second doses was 1.76 per 100,000 persons (95% confidence interval [CI], 1.33 to 2.19), with the largest difference among male recipients between the ages of 16 and 19 years (difference, 13.73 per 100,000 persons; 95% CI, 8.11 to 19.46). As compared with the expected incidence based on historical data, the standardized incidence ratio was 5.34 (95% CI, 4.48 to 6.40) and was highest after the second dose in male recipients between the ages of 16 and 19 years (13.60; 95% CI, 9.30 to 19.20). The rate ratio 30 days after the second vaccine dose in fully vaccinated recipients, as compared with unvaccinated persons, was 2.35 (95% CI, 1.10 to 5.02); the rate ratio was again highest in male recipients between the ages of 16 and 19 years (8.96; 95% CI, 4.50 to 17.83), with a ratio of 1 in 6637. The incidence of myocarditis, although low, increased after the receipt of the BNT162b2 vaccine, particularly after the second dose among young male recipients. The clinical presentation of myocarditis after vaccination was usually mild.

Immature dendritic cells (iDCs) do not mature after uptake of apoptotic cells and may play a role in the induction of peripheral tolerance to self antigens derived from apoptotic material. The integrins, αvβ3, αvβ5, and the scavenger receptor, CD36, have been shown to mediate uptake of apoptotic cells by iDCs. However, it is not known whether the complement system, also takes part in this process. In this study we investigated the ability of iDCs to bind to apoptotic cells opsonized by iC3b. Monocyte-derived dendritic cells were offered apoptotic Jurkat cells opsonized by autologous iC3b and labeled with 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl-indocarbocyanineperchlorate. A significant increase (P < 0.001) in the amount of cleared apoptotic cells was seen at low ratios. Despite increased efficiency of uptake, interaction between iC3b-opsonized apoptotic cells and iDCs down-regulated the expression of major histocompatibility complex class II, CD86, CC chemokine receptor (CCR)2, CCR5, and β2-integrins (P < 0.001), and up-regulated expression of CCR7 (P < 0.001). In addition, iDC maturation responses to CD40L and lipopolysaccharide were significantly inhibited. We conclude that opsonization of apoptotic cells by iC3b induces tolerant iDCs that are able to migrate to lymph nodes.

Inefficient and abnormal clearance of apoptotic cells (efferocytosis) contributes to systemic autoimmune disease in humans and mice, and inefficient chromosomal DNA degradation by DNAse II leads to systemic polyarthritis and a cytokine storm. In contrast, efficient clearance allows immune homeostasis, generally leads to a non-inflammatory state for both macrophages and dendritic cells (DCs), and contributes to maintenance of peripheral tolerance. As many as 3×108 cells undergo apoptosis every hour in our bodies, and one of the primary ‘eat me’ signals expressed by apoptotic cells is phosphatidylserine. Apoptotic cells themselves are major contributors to the ‘anti-inflammatory’ nature of the engulfment process, some by secreting thrombospondin-1 or AMP and possibly other immune modulating ‘calm-down’ signals that interact with macrophages and DCs. Apoptotic cells also produce ‘find me’ and ‘tolerate me’ signals to attract and immune modulate macrophages and DCs that express specific receptors for some of these signals. Neither macrophages or DCs are uniform and each cell type may variably express membrane proteins that function as receptors for phosphatidylserine or for opsonins like complement or opsonins that bind to phosphatidylserine, like protein S and growth arrest-specific 6. Macrophages and DCs also express scavenger receptors, CD36, and integrins that function via bridging molecules such as thrombospondin 1 or milk fat globule-EGF factor 8 protein and that differentially engage in various multi-ligand interactions between apoptotic cells and phagocytes. In this review, we describe the anti-inflammatory and pro-homeostatic nature of apoptotic cell interaction with the immune system. We do not review some forms of immunogenic cell death. We summarize the known apoptotic cell signaling events in macrophages and DCs that are related to toll-like receptors, nuclear factor kappa B (NF-κB), inflammasome, the lipid activated nuclear receptors, Tyro3, Axl, and Mertk receptors, as well as induction of signal transducer and activator of transcription 1(STAT1) and suppressor of cytokine signaling (SOCS) that lead to immune system silencing and DC tolerance. These properties of apoptotic cells are the mechanisms that enable their successful use as therapeutic modalities in mice and humans in various autoimmune diseases, organ transplantation, graft versus host disease, and sepsis.

Myocarditis associated with messenger RNA (mRNA) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) predominantly affects male adolescents and young male adults (14 to <30 years of age) and typically occurs after receipt of the second vaccine dose.^1,2 In adults with critical coronavirus disease 2019 (Covid-19) and in cases of multisystem inflammatory syndrome in children (MIS-C), we recently discovered neutralizing autoantibodies targeting the endogenous interleukin-1 receptor antagonist (IL-1RA), which inhibits interleukin-1 signaling and inflammation.^3,4

In recent years, it has become apparent that the removal of apoptotic cells by macrophages and DC is not only noninflammatory, but also immune‐inhibitory, in most although not all circumstances. Complement may be involved in the uptake of apoptotic cells via direct binding of bridging factors in some physiological circumstances, by opsonization and engagement of the complement receptors. In the current study, we use a complement‐dependent system of apoptotic cell clearance by human‐derived macrophages and DC. Using a luciferase reporter gene and measuring immune response to non‐opsonic zymosan, we show that iC3b‐apoptotic cells induce NF‐κB inhibition in response to zymosan and LPS at the nuclear translocation, transcriptional and post‐transcriptional levels, leading to profound inhibition of proinflammatory cytokines. In addition, interaction with iC3b‐opsonized apoptotic cells is characterized by macrophage secretion of IL‐10 and lack of TGF‐β secretion. In conclusion, in cells with iC3b receptors, opsonized apoptotic cells mediate a distinct anti‐inflammatory response and transcriptional NF‐κB‐dependent blockage.

Sepsis has no proven pharmacologic treatment other than appropriate antibiotic agents, fluids, vasopressors as needed, and possibly corticosteroids. It is generally initiated mainly by the simultaneous recognition by various components of the innate immune system of either pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs). In the current study, we employed the murine cecal ligation and puncture (CLP) model for sepsis to evaluate the effect of post-CLP infusion of apoptotic cells (Allocetra-OTS) on a CLP severe sepsis model. Cardiovascular evaluation, acute kidney injury (AKI), acute liver injury (ALI), and hematological and metabolic function were evaluated. Cytokine and chemokine profiles were measured by Multiplex ELISA and mitochondrial function, and glycolysis by Seahorse. The Murine Sepsis Score (MSS) was used for disease severity definition. CLP mice had low blood pressure, poor cardiac output, and lung dysfunction, as well as AKI, ALI, and thrombocytopenia, which correlated with the MSS and corresponded to a cytokine/chemokine storm. Apoptotic cell administration markedly improved the cytokine and chemokine storm and restored the impaired mitochondrial and glycolytic function in white blood cells leading to increased survival, from 6 to 60% (P < 0.0001), together with a significant improvement in organ dysfunction. We conclude that the deleterious immune response in CLP-induced sepsis can be successfully modified by apoptotic cell infusion.