Cross-reactive CD4+ T cells enhance SARS-CoV-2 immune responses upon infection and vaccination.

The functional relevance of preexisting cross-immunity to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a subject of intense debate. Here, we show that human endemic coronavirus (HCoV)­reactive and SARS-CoV-2­cross-reactive CD4+ T cells are ubiquitous but decrease with age. We identified a universal immunodominant coronavirus-specific spike peptide (S816-830) and demonstrate that preexisting spike- and S816-830­reactive T cells were recruited into immune responses to SARS-CoV-2 infection and their frequency correlated with anti­SARS-CoV-2-S1-IgG antibodies. Spike­cross-reactive T cells were also activated after primary BNT162b2 COVID-19 messenger RNA vaccination and displayed kinetics similar to those of secondary immune responses. Our results highlight the functional contribution of preexisting spike­cross-reactive T cells in SARS-CoV-2 infection and vaccination. Cross-reactive immunity may account for the unexpectedly rapid induction of immunity after primary SARS-CoV-2 immunization and the high rate of asymptomatic or mild COVID-19 disease courses.

Effects of 5-aza-2´-deoxycytidine on primary human chondrocytes from osteoarthritic patients.

Chondrocytes, comparable to many cells from the connective tissue, dedifferentiate and end up in a similar fibroblastoid cell type, marked by the loss of the specific expression pattern. Here, chondrocytes isolated from osteoarthritic (OA) patients were investigated. The OA chondrocytes used in this work were not affected by the loss of specific gene expression upon cell culture. The mRNA levels of known cartilage markers, such as SOX5, SOX6, SOX9, aggrecan and proteoglycan 4, remained unchanged. Since chondrocytes from OA and healthy tissue show different DNA methylation patterns, the underlying mechanisms of cartilage marker maintenance were investigated with a focus on the epigenetic modification by DNA methylation. The treatment of dedifferentiated chondrocytes with the DNA methyltransferase inhibitor 5-aza-2´-deoxycytidine (5-aza-dC) displayed no considerable impact on the maintenance of marker gene expression observed in the dedifferentiated state, while the chondrogenic differentiation capacity was compromised. On the other hand, the pre-cultivation with 5-aza-dC improved the osteogenesis and adipogenesis of OA chondrocytes. Contradictory to these effects, the DNA methylation levels were not reduced after treatment for four weeks with 1 µM 5-aza-dC. In conclusion, 5-aza-dC affects the differentiation capacity of OA chondrocytes, while the global DNA methylation level remains stable. Furthermore, dedifferentiated chondrocytes isolated from late-stage OA patients represent a reliable cell source for in vitro studies and disease models without the need for additional alterations.