ACE2-independent entry factors for SARS-CoV-2 infection and immune activation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), remains a major public health threat, particularly in vulnerable populations. SARS-CoV-2 spike proteins interact with the human angiotensin-converting enzyme 2 (ACE2) receptor, together with accessory molecules that facilitate viral entry, through its spike receptor-binding domain (RBD). Although ACE2 is the primary receptor required for viral replication, its expression patterns do not fully correlate with viral distribution or tissue pathology. Moreover, SARS-CoV-2 has been shown to infect cells and tissues lacking detectable ACE2 expression. Viral entry via ACE2-independent pathways may also confer resistance to some monoclonal antibodies (Abs) targeting the spike RBD that block ACE2-mediated binding. These observations highlight the potential significance of ACE2-independent entry factors in SARS-CoV-2 infection, particularly in vaccinated individuals with Abs directed against ACE2-dependent viral entry. In this review, we discuss the emerging roles of ACE2-independent entry factors in SARS-CoV-2 infection and the immune responses. These factors include CD147, AXL, CD169/Siglec-1, CD209L, CD209, CLEC4G, ASGR1, LDLRAD3, TMEM30A, TMEM106B, transferrin receptor 1, GPR78, integrin α5β1, KREMEN1, LFA-1, and CD4. While ACE2 remains central to viral replication, ACE2-independent entry appears sufficient to elicit immune responses. Therefore, future investigations are warranted to elucidate the roles of ACE2-independent mechanisms in immune-mediated pathology and viral evolution, independent of immune pressure targeting ACE2-mediated entry in previously infected or vaccinated individuals.