It is crucial to analyze the consequences of repeated messenger RNA (mRNA)-based COVID-19 vaccinations on SARS-CoV-2 spike receptor binding domain (RBD)-specific immunoglobulin (Ig)G subclass and the possible causal relationship with breakthrough infection. We examined the longitudinal kinetics of RBD-specific IgG subclass antibodies in sera after receiving the second, third, and fourth doses of mRNA-based COVID-19 vaccines in Japanese healthcare workers. Anti-RBD IgG subclass in sera of patients with COVID-19-infected who had not received the COVID-19 vaccine were also examined. We compared anti-RBD IgG subclass antibody titers in the serum of pre-breakthrough-infected vaccinees and non-infected vaccinees. The seropositivity of anti-RBD IgG4 after the vaccination was 6.76% at 1 month after the second dose, gradually increased to 50.5% at 6 months after the second dose, and reached 97.2% at 1 month after the third dose. The seropositivity and titers of anti-RBD IgG1/IgG3 quickly reached the maximum at 1 month after the second dose and declined afterward. The elevated anti-RBD IgG4 Ab levels observed after repeated vaccinations were unlikely to increase the risk of breakthrough infection. Repeated vaccinations induce delayed but drastic increases in anti-RBD IgG4 responses. Further functional investigations are needed to reveal the magnitude of the high contribution of spike-specific IgG4 subclasses after repeated mRNA-based COVID-19 vaccinations.

This study aims to examine whether the parenterally administered mRNA-based COVID-19 vaccines can induce sufficient mucosal-type IgA responses to prevent SARS-CoV-2 transmission. We examined the longitudinal kinetics of SARS-CoV-2 spike RBD-specific IgA and IgG responses in sera of Japanese healthcare workers (HCWs) after receiving two doses and the third dose of BNT162b2 mRNA vaccines. During the prospective cohort study, Omicron breakthrough infections occurred in 62 participants among 370 HCWs who had received triple doses of the vaccine. Pre-breakthrough sera of infected HCWs and non-infected HCWs were examined for the levels of anti-RBD IgA and IgG titers. The seropositivity of anti-RBD IgA at 1 M after the second vaccine (2D-1M) and after the third dose (3D-1M) was 65.4% and 87.4%, respectively, and wanes quickly. The boosting effect on anti-RBD Ab titers following breakthrough infections was more notable for anti-RBD IgA than for IgG. There were partial cause-relationships between the lower anti-RBD IgA or IgG at pre-breakthrough sera and the breakthrough infection. Parenterally administered COVID-19 vaccines do not generate sufficient mucosal-type IgA responses despite strong systemic IgG responses to SARS-CoV-2. These results demonstrate the necessity and importance of reevaluating vaccine design and scheduling to efficiently increase oral or respiratory mucosal immunity against SARS-CoV-2.

Neutrophils are among the first cells to be recruited to the lungs during Chlamydia pneumoniae infection in mouse models; however, their regulatory functions are not yet fully understood. This study examined the mechanisms and significance of IL-10-producing neutrophils throughout C. pneumoniae pulmonary infection in C57BL/6 mice. Our findings revealed that infection with C. pneumoniae induces IL-10 secretion in bone marrow-derived neutrophils, depending on Toll-like receptor 2 (TLR2) activation. This process involves TLR2-dependent mitochondrial reactive oxygen species (ROS) production, which triggers the endoplasmic reticulum (ER) stress pathway, including IRE1α and subsequent Xbp1 splicing. Inhibition of this pathway or depletion of neutrophils (using the 1A8 monoclonal antibody) significantly reduces IL-10 levels in bronchoalveolar lavage fluid (BALF) in vivo. Conversely, the absence of IL-10-producing neutrophils, whether through depletion or TLR2 deficiency, leads to increased IL-12p70 and IFN-γ-positive NK cells, along with decreased regulatory T cells and M2-like macrophages. This results in a lower bacterial burden in the lungs but causes more severe pulmonary damage and decreased survival rates. These findings highlight that IL-10 produced by neutrophils via the TLR2-mitochondrial ROS–ER stress pathway is essential for modulating pulmonary immune responses and maintaining immune homeostasis during C. pneumoniae infection, thereby preventing excessive inflammation and tissue damage.

Analysis of longitudinal dynamics of humoral immune responses to the BNT162b2 COVID-19 vaccine might provide useful information to predict the effectiveness of BNT162b2 in preventing SARS-CoV-2 infection. Herein, we measure anti-RBD IgG at 1, 3 and 6 months (M) after the second dose of BNT162b2, and at 1 M after a third dose of BNT162b2 vaccination in 431 COVID-19-naïve healthcare workers (HCWs) in Japan. All HCWs mounted high-anti-RBD IgG responses after the two-dose regimen of BNT162b2 vaccinations. Older persons and males presented lower anti-RBD IgG responses than younger adults and females, respectively. The decay in anti-RBD IgG started from 1 M after the second dose of BNT162b2 and anti-RBD IgG titers dropped to nearly one-tenth at 6 M after the second vaccination. Subsequently, the participants received a third dose of BNT162b2 at 8 M after the second dose of BNT162b2 vaccine. Anti-RBD antibody titers 1 M after the third dose of BNT162b2 increased seventeen times that of 6 M after the second dose, and was twice higher than the peak antibody titers at 1 M after the second dose of vaccination. The negative effect of age for the male gender on anti-RBD IgG antibody titers was not observed at 1 M after the third dose of BNT162b2 vaccine. There were no notable adverse events reported, which required hospitalization in these participants. These results suggest that the third dose of BNT162b2 safely improves humoral immunity against SARS-CoV-2 with no major adverse events.

Fatty acid–binding protein 4 (FABP4) is a critical immune-metabolic modulator, mainly expressed in adipocytes and macrophages, secreted from adipocytes in association with lipolysis, and plays essential pathogenic roles in cardiovascular and metabolic diseases. We previously reported Chlamydia pneumoniae infecting murine 3T3-L1 adipocytes and causing lipolysis and FABP4 secretion in vitro. However, it is still unknown whether C. pneumoniae intranasal lung infection targets white adipose tissues (WATs), induces lipolysis, and causes FABP4 secretion in vivo. In this study, we demonstrate that C. pneumoniae lung infection causes robust lipolysis in WAT. Infection-induced WAT lipolysis was diminished in FABP4−/− mice or FABP4 inhibitor–pretreated wild-type mice. Infection by C. pneumoniae in wild-type but not FABP4−/− mice induces the accumulation of TNF-α– and IL-6–producing M1-like adipose tissue macrophages in WAT. Infection-induced WAT pathology is augmented by endoplasmic reticulum (ER) stress/the unfolded protein response (UPR), which is abrogated by treatment with azoramide, a modulator of the UPR. C. pneumoniae lung infection is suggested to target WAT and induce lipolysis and FABP4 secretion in vivo via ER stress/UPR. FABP4 released from infected adipocytes may be taken up by other neighboring intact adipocytes or adipose tissue macrophages. This process can further induce ER stress activation and trigger lipolysis and inflammation, followed by FABP4 secretion, leading to WAT pathology. A better understanding of the role of FABP4 in C. pneumoniae infection–induced WAT pathology will provide the basis for rational intervention measures directed at C. pneumoniae infection and metabolic syndrome, such as atherosclerosis, for which robust epidemiologic evidence exists.