CD147 Interacts with SARS-CoV-2 Spike Protein to Facilitate Infection; SARS-CoV-2-Reactive IgA is Found in Breastmilk

ACE2 is well established as the primary target for viral attachment and entry for both SARS-CoV-1 and SARS-CoV-2 (Li et al., 2005; Zhang et al., 2020). In addition, several other molecules have been discovered to play an accessory role in permitting viral entry into host cells. One of these accessory proteins is a molecule called TMPRSS2, a serine protease that acts to prime the viral spike (S) protein of SARS-CoV-1 prior to interaction with ACE2 and was also found to be implicated in the attachment of SARS-CoV-2 (Hoffman et al., 2020). More recently, CD147 was found to be a novel mediator of viral attachment in SARS-CoV-2 infection. CD147 is a membrane-spanning glycoprotein that is prominently associated with tumor development, protection from plasmodium invasion, and viral infection: it has been shown to inhibit signaling pathways of the tumor suppressor p53, block red blood cell infection of the protozoan that causes malaria P. falciparum, and facilitate HIV invasion (Huang et al., 2014; Zhang et al., 2017; Pushkarsky et al., 2001). Previously, Wang et al. showed that CD147 facilitates SARS-CoV-1 entry, which could be inhibited by CD147-antagonist peptide-9 in the HEK293T cell line (Chen et al., 2005). Here, they first demonstrated that humanized anti-CD147 monoclonal antibody Meplazumab competitively inhibited binding of the S protein and CD147 – compared to controls, CD147-blocked cells displayed significantly less infection by SARS-CoV-2. Additionally, Surface Plasmon Resonance (SPR, Top figure 1A) and co-immunoprecipitation with anti-CD147 and anti-S protein (Top figure 1B) confirmed the interaction of the two molecules with micromolar affinity (Kd = 1.85 x 10-7 M); and competitive direct ELISA (Top figure 1C-D) confirmed a dose-dependent interaction and its inhibition by Meplazumab. Follow-up observation by electron microscopy in infected Vero E6 cells (Bottom figure 1A-C) demonstrated significant co-localization of CD147 and S protein in viral inclusion bodies (Wang et al., 2020). The discovery of novel routes of entry provides unique opportunities for supplementing the molecular arsenal against SARS-CoV-2.

Convalescent plasma screening for IgG and IgM against pathogens is an important method for [relatively] quick identification of potentially neutralizing epitopes on those pathogens. However, of growing import is the transmission of the potentially protective IgA from mothers previously infected with SARS-CoV-2 to their babies through breastfeeding. In a recent study, fifteen milk samples were obtained from previously infected donors and compared to post-outbreak controls (the post-infection samples were obtained between 14 and 30 days after symptoms abated in these patients). In twelve of the fifteen samples, purified IgA demonstrated significant reactivity to the receptor-binding domain (RBD) of the S protein of SARS-CoV-2. Overall, the optical density measurements for the COVID-19-recovered patients were significantly greater than control measurements for IgA, IgG and secretory-type antibodies specific for the RBD (Fox et al., 2020). Growing support for the intestine’s role in proper immune system development and functioning has lent credence to a connection between this environment and that of the IgA-protected mucosa of the mammary glands – the so-called “entero-mammary link”. The intestines, being a mucosal environment, are predominant sites of IgA production and secretion by intraepithelial B cells proliferating in mesenteric lymph nodes. The authors postulate that IgA specific for SARS-CoV-2 may have originated in the gut, as previous mammary IgA has been found to exhibit specificity for enteric antigens (Seppo et al., 2017). This work suggests an important role for IgA in the immune response against SARS-CoV-2.

Written by: Parker Davis
Edited by: Jina Zhou and Esther Melamed
5/18/2020