Compared with other coronaviruses, SARS-CoV-2 is highly transmissible and pathogenic. The virus has a short production phase and a predominantly latent infection, despite expressing a limited number of proteins, some of which exert immunomodulatory roles in preventing recognition by the host immune surveillance system [
25,
26]. For example, SARS-CoV-2 ORF6 blocks type II interferon-mediated immune responses and promote viral infection and replication [
27]. In addition, angiotensin-converting enzyme 2 (ACE2) or major histocompatibility complex (MHC) alleles are associated with viral titers [
20,
28]. MHC molecules are involved in antigen recognition and presentation in the immune response process. After the virus enters the body, MHC class I can deliver the peptide chain of the viral membrane to the outside of the cell for T cells to recognize and eliminate the virus [
29]. Analysis of gene expression profiles of COVID-19 patients and SARS-CoV-2 infected epithelial cell lines showed that SARS-CoV-2 infection inhibits the activation of the MHC class I pathway, thereby facilitating viral escape from host immune surveillance [
27]. The expression phenotype and genetic characteristics of host MHC molecules are closely related to viral infection. In this study, we examined differences in pathogenicity across mice infected with SARS-CoV-2/BMA8 and revealed an important role for MHC-mediated T-cell responses in host pathogenicity.
Notably, the MHC class I of C57BL/6N mice was H-2K
b, whereas that of BALB/c was H-2K
d. Therefore, we assume that the MHC class I/H-2K
b haplotype confers better immunity to C57BL/6N. Indeed, MHC inhibition promoted infection and pathogenicity of the SARS-CoV-2 mouse-adapted strain BMA8 in non-susceptible C57BL/6N mice. This also indicates the important role of MHC in the SARS-CoV-2 lifecycle and host immune response. Viral transmission to MHC class I mismatched individuals may result in the reversion of these mutations to restore viral fitness [
30‐
32]. Therefore, we speculated that SARS-CoV-2 might evolve by selecting individual MHC class I-associated mutations and their reversion after multiple transmissions among individuals with highly diverse MHC class I haplotype. However, the specific mechanism needs to be further explored.
Inherent phenotypes of viruses and host characteristics are closely related to their pathogenicity, including differences in susceptibility and host immune responses [
33]. The lymphocytes (T cells and B cells), which differentiate into specific effector cells by regulating cytokines and chemokines, are recruited to exert their biological effects at the site of infection initiation and vice versa [
34]. T lymphocytes are important sources of many cytokines/chemokines and express various molecular receptors. For example, the CD4
+ T cells differentiate into Th1 cells after stimulation with interleukin-12 (IL-12) and produce interferon-gamma (IFN-γ) to activate immune cells, thereby triggering response signaling and defense mechanisms against intracellular pathogens [
35]. In the process of virus infection, CD8
+ T cell-mediated immune responses are responsible for the clearance of viruses, such as influenza virus [
36], respiratory syncytial virus [
37] and severe acute respiratory syndrome coronavirus [
38], and can even provide protection against secondary infections [
39,
40]. Many studies also have shown that CD8
+ T cell responses exert strong inhibitory pressure on SARS-CoV-2 replication [
19,
33,
41]. Additionally, IL-6 induces the differentiation of CD8
+ T cells to eliminate the virus by splitting the infected cells [
42]. This suggests that the consumption of T cells may be an important reason for the decrease of lymphocytes in COVID-19 patients. Similarly, neutrophils and macrophages can be recruited to the injured area in the presence of chemokines, respectively, to recruit more immune cells to resist pathogens [
43]. Clinical studies have shown that patients with COVID-19 often have thrombocytopenia syndrome, and platelets are activated to release downstream cytokines (or chemokines) to participate in immune regulation and response [
44]. This is mainly because the complex relationship between cytokines (or chemokines) and their receptors enables them to be rapidly replenished in microenvironments, leading to the continued development of inflammatory response. Furthermore, inflammation and virus infection impact the function of platelets, causing clots to aggregate more rapidly, explaining increased thrombosis after SARS-CoV-2 infection [
45]. Clinical studies have also found that lymphocytopenia and pro-inflammatory cytokine storm are more acute in COVID-19 severe patients than in mild patients and correlate with disease severity [
46]. Consistently, our results also showed that the number of lymphocytes (especially CD8
+ T cells) and platelets were significantly reduced after SARS-CoV-2 infection in a mouse model and that BALB/c mice with high cytokine expression had a higher degree of pathogenesis than C57BL/6N mice. Here, we detected the changes in the number of CD8
+ T cells before and after infection, in order to better illustrate the potential mechanism of MHC affecting host pathogenicity, we also need to dynamically analyze the changes of systemic immune response (including NK cells, etc.) caused by the differences in host MHC molecules in future.