Lower Levels of ABO Anti-A and Anti-B of IgM, IgG and IgA Isotypes in the Serum but Not the Saliva of COVID-19 Convalescents.

Individuals with ABO type O, naturally possessing anti-A and anti-B antibodies in their serum, are underrepresented among patients infected with SARS-CoV-2 compared with healthy controls. The ABO antibodies might play a role in the viral transmission. Therefore, we aimed to quantify anti-A/anti-B, including their subclasses IgM, IgG and IgA, in the serum and saliva of Caucasians (n = 187) after mild COVID-19 to compare them with individuals who had never been infected with SARS-CoV-2. Two samples were collected within two months after the diagnosis (median days: 44) and two months later. ABO antibodies were determined by flow cytometry. Additionally, total IgA in saliva and antibodies specific to SARS-CoV-2 were tested by ELISA. COVID-19 convalescents had significantly lower levels of anti-A/anti-B IgM, IgG and IgA in their serum than control subjects (p < 0.001). Interestingly, no significant differences were observed in saliva. ABO antibody levels remained stable over the period considered. No relation of ABO to the level of SARS-CoV-2-specific antibodies was observed. Total IgA was lower in convalescents than in controls (p = 0.038). Whereas ABO antibodies in the saliva may not contribute to the pathogenesis of COVID-19, individual pre-existing high serum concentrations of anti-A/anti-B may have a protective effect against SARS-CoV-2 infection.

Distinct chromatin signature of histone H3 variant H3.3 in human cells.

Actively transcribed regions of the genome have been found enriched for the histone H3 variant H3.3. This variant is incorporated into nucleosomes throughout the cell cycle whereas the canonical isoforms are predominately deposited in association with replication. In order to obtain a global picture of the deposition pattern at the single cell level we expressed H3.3 in both normal and malignant human cells and analyzed nuclei using conventional and structured illumination imaging (SIM). We found that the distribution pattern of H3.3 in interphase differs from that of the canonical histone H3 variants and this difference is conveyed to mitotic chromosomes which display a distinct H3.3 banding pattern. Histone H3.3 localization positively correlated with markers for transcriptionally active chromatin and, notably, H3.3 was almost completely absent from the inactive X chromosome. Collectively, our data show that histone variant H3.3 occupies distinct intranuclear chromatin domains and that these genomic loci are associated with gene expression.