A Fail-safe Mechanism for Negative Selection of Isotype-switched B Cell Precursors Is Regulated by the Fas/FasL Pathway

In B lymphocytes, immunoglobulin (Ig)M receptors drive development and construction of naive repertoire, whereas IgG receptors promote formation of the memory B cell compartment. This isotype switching process requires appropriate B cell activation and T cell help. In the absence of T cell help, activated B cells undergo Fas-mediated apoptosis, a peripheral mechanism contributing to the establishment of self-tolerance. Using Igμ-deficient μMT mouse model, where B cell development is blocked at pro-B stage, here we show an alternative developmental pathway used by isotype-switched B cell precursors. We find that isotype switching occurs normally in B cell precursors and is T independent. Ongoing isotype switching was found in both normal and μMT B cell development as reflected by detection of IgG1 germline and postswitch transcripts as well as activation-induced cytidine deaminase expression, resulting in the generation of IgG-expressing cells. These isotype-switched B cells are negatively selected by Fas pathway, as blocking the Fas/FasL interaction rescues the development of isotype-switched B cells in vivo and in vitro. Similar to memory B cells, isotype-switched B cells have a marginal zone phenotype. We suggest a novel developmental pathway used by isotype-switched B cell precursors that effectively circumvents peripheral tolerance requirements. This developmental pathway, however, is strictly controlled by Fas/FasL interaction to prevent B cell autoimmunity

IgG1 B cell receptor signaling is inhibited by CD22 and promotes the development of B cells whose survival is less dependent on Igα/β

We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) γ1 or μ heavy chains. Progenitor cells expressing γ1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro– to pre–B cell transition. Accordingly, γ1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Igα cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca2+ response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca2+ response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the γ1 chain can exert a unique signaling function that can partially replace that of the Igα/β heterodimer in B cell maintenance and may contribute to memory B cell physiology

LMP1 signaling can replace CD40 signaling in B cells in vivo and has unique features of inducing class-switch recombination to IgG1.

The Epstein-Barr virus (EBV) protein LMP1 is considered to be a functional homologue of the CD40 receptor. However, in contrast to the latter, LMP1 is a constitutively active signaling molecule. To compare B cell-specific LMP1 and CD40 signaling in an unambiguous manner, we generated transgenic mice conditionally expressing a CD40/LMP1 fusion protein, which retained the LMP1 cytoplasmic tail but has lost the constitutive activity of LMP1 and needs to be activated by the CD40 ligand. We show that LMP1 signaling can completely substitute CD40 signaling in B cells, leading to normal B-cell development, activation, and immune responses including class-switch recombination, germinal center formation, and somatic hypermutation. In addition, the LMP1-signaling domain has a unique property in that it can induce class-switch recombination to IgG1 independent of cytokines. Thus, our data indicate that LMP1 has evolved to imitate T-helper cell function allowing activation, proliferation, and differentiation of EBV-infected B cells independent of T cells

Antiviral Drug Discovery for the Treatment of COVID-19 Infections

The coronavirus disease 2019 (COVID-19) pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a recently emerged human coronavirus. COVID-19 vaccines have proven to be successful in protecting the vaccinated from infection, reducing the severity of disease, and deterring the transmission of infection. However, COVID-19 vaccination faces many challenges, such as the decline in vaccine-induced immunity over time, and the decrease in potency against some SARS-CoV-2 variants including the recently emerged Omicron variant, resulting in breakthrough infections. The challenges that COVID-19 vaccination is facing highlight the importance of the discovery of antivirals to serve as another means to tackle the pandemic. To date, neutralizing antibodies that block viral entry by targeting the viral spike protein make up the largest class of antivirals that has received US FDA emergency use authorization (EUA) for COVID-19 treatment. In addition to the spike protein, other key targets for the discovery of direct-acting antivirals include viral enzymes that are essential for SARS-CoV-2 replication, such as RNA-dependent RNA polymerase and proteases, as judged by US FDA approval for remdesivir, and EUA for Paxlovid (nirmatrelvir + ritonavir) for treating COVID-19 infections. This review presents an overview of the current status and future direction of antiviral drug discovery for treating SARS-CoV-2 infections, covering important antiviral targets such as the viral spike protein, non-structural protein (nsp) 3 papain-like protease, nsp5 main protease, and the nsp12/nsp7/nsp8 RNA-dependent RNA polymerase complex