SARS-CoV-2 has infected millions of people and is on a trajectory to kill more than one million globally. Virus entry depends on the receptor-binding domain (RBD) of the spike protein. Although previous studies demonstrated anti-spike and -RBD antibodies as essential for protection and convalescent plasma as a promising therapeutic option, little is known about the immunoglobulin (Ig) isotypes capable of blocking virus entry. Here, we studied spike- and RBD-specific Ig isotypes in plasma/sera from two acutely infected and 29 convalescent individuals. Spike- and RBD-specific IgM, IgG1, and IgA1 antibodies were produced by all or nearly all subjects at varying levels and detected at 7-8 days post-disease onset. IgG2, IgG3, IgG4, and IgA2 were also present but at much lower levels. All samples also displayed neutralizing activity. IgM, IgG, and IgA were capable of mediating neutralization, but neutralization titers correlated better with binding levels of IgM and IgA1 than IgG.

It has been proposed that CD6, an important regulator of T cells, functions by interacting with its currently identified ligand, CD166, but studies performed during the treatment of autoimmune conditions suggest that the CD6-CD166 interaction might not account for important functions of CD6 in autoimmune diseases. The antigen recognized by mAb 3A11 has been proposed as a new CD6 ligand distinct from CD166, yet the identity of it is hitherto unknown. We have identified this CD6 ligand as CD318, a cell surface protein previously found to be present on various epithelial cells and many tumor cells. We found that, like CD6 knockout (KO) mice, CD318 KO mice are also protected in experimental autoimmune encephalomyelitis. In humans, we found that CD318 is highly expressed in synovial tissues and participates in CD6-dependent adhesion of T cells to synovial fibroblasts. In addition, soluble CD318 is chemoattractive to T cells and levels of soluble CD318 are selectively and significantly elevated in the synovial fluid from patients with rheumatoid arthritis and juvenile inflammatory arthritis. These results establish CD318 as a ligand of CD6 and a potential target for the diagnosis and treatment of autoimmune diseases such as multiple sclerosis and inflammatory arthritis.C D6 is a marker of T cells and an important T-cell regulator (1). Recent genome-wide association studies also identified CD6 as a risk gene for multiple sclerosis (MS) (2-5), an autoimmune disease in which T cells play a vital role in the pathogenesis. CD6 is composed of three extracellular domains (domains 1, 2, and 3), and it functions by interacting with its ligand(s) (6). The domain 3 of CD6 has been shown to be the site that the identified CD6 ligand, CD166, also known as ALCAM (activated leukocyte cell adhesion molecule), binds to (7). However, anti-CD166 antibodies only partially blocked the binding of thymic epithelial cells to CD6-overexpressing COS cells, and mAbs blocking CD6-CD166 interactions do not abolish CD6 function (8, 9). Itolizumab, an anti-CD6 mAb developed in Cuba and approved in India for treating psoriasis, reduces pathogenic T-cell responses in patients with psoriasis, but this mAb binds to domain 1 of CD6 instead of domain 3, and it does not interfere with the CD6-CD166 interaction. Interestingly, UMCD6, a mouse antihuman CD6 mAb that we found highly effective in treating encephalomyelitis (EAE) in CD6 humanized mice, also fails to block the CD6-CD166 interaction. All these studies suggest the existence of an additional CD6 ligand, other than CD166, that binds to domain 1 of CD6, and could be critical for CD6 function in autoimmune conditions. Further studies using a CD6 fusion protein as a bait to pull down CD6-binding proteins from synovial fibroblast surface proteins showed the binding of three polypeptides (10). One of these polypeptides was identified as CD166, and the identities of the other two were unknown (11). A mAb termed 3A11 was developed, and the antigen recognized by this mAb was identified as the new ligand of ...

Background SARS-CoV-2 has infected millions of people globally. Virus infection requires the receptor-binding domain (RBD) of the spike protein. Although studies have demonstrated anti-spike and -RBD antibodies to be protective in animal models, and convalescent plasma as a promising therapeutic option, little is known about immunoglobulin (Ig) isotypes capable of blocking infection. Methods We studied spike- and RBD-specific Ig isotypes in convalescent and acute plasma/sera using a multiplex bead assay. We also determined virus neutralization activities in plasma, sera, and purified Ig fractions using a VSV pseudovirus assay. Results Spike- and RBD-specific IgM, IgG1, and IgA1 were produced by all or nearly all subjects at variable levels and detected early after infection. All samples displayed neutralizing activity. Regression analyses revealed that IgM and IgG1 contributed most to neutralization, consistent with IgM and IgG fractions’ neutralization potency. IgA also exhibited neutralizing activity, but with lower potency. Conclusion IgG, IgM and IgA are critical components of convalescent plasma used for COVID-19 treatment.

Mucosal-associated invariant T (MAIT) cells help combat opportunistic infections. Thus, MAIT cells are of interest in HIV/SIV vaccination and infection. We investigated MAIT cell dynamics and function in rhesus macaque blood and bronchoalveolar lavage (BAL) following mucosal adenovirus (Ad)-SIV recombinant priming, intramuscular SIV envelope boosting and infection following repeated low-dose intravaginal SIV exposures. Increased frequencies of blood MAIT cells over the course of vaccination were observed, which were maintained even 12-weeks post-SIV infection. BAL MAIT cells only increased after the first Ad immunization. Vaccination increased MAIT cell levels in blood and BAL expressing the antiviral cytokine IFN-γ and TNF-α and the proliferation marker Ki67. Upon T cell-specific α-CD3, α-CD28 stimulation, MAIT cells showed a greater capacity to secrete cytokines/chemokines associated with help for B cell activation, migration and regulation compared to CD3+MR1− cells. Culture of MAIT cell supernatants with B cells led to greater tissue like memory B cell frequencies. MAIT cell frequencies in blood and BAL correlated with SIV-specific antibody levels in rectal secretions and with SIV-specific tissue resident memory B cells. Overall, SIV vaccination influenced MAIT cell frequency and functionality. The potential for MAIT cells to provide help to B cells was evident during both vaccination and infection.