Immunopathology and Immunopathogenesis of COVID-19, what we know and what we should learn

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) directly interacts with host's epithelial and immune cells, leading to inflammatory response induction, which is considered the hallmark of infection. The host immune system is programmed to facilitate the clearance of viral infection by establishing a modulated response. However, SARS-CoV-2 takes the initiative and its various structural and non-structural proteins directly or indirectly stimulate the uncontrolled activation of injurious inflammatory pathways through interaction with innate immune system mediators. Upregulation of cell-signaling pathways such as mitogen-activate protein kinase (MAPK) in response to recognition of SARS-CoV-2 antigens by innate immune system receptors mediates unbridled production of proinflammatory cytokines and cells causing cytokine storm, tissue damage, increased pulmonary edema, acute respiratory distress syndrome (ARDS), and mortality. Moreover, this acute inflammatory state hinders the immunomodulatory effect of T helper cells and timely response of CD4⁺ and CD8⁺ T cells against infection. Furthermore, inflammation-induced overproduction of Th17 cells can downregulate the antiviral response of Th1 and Th2 cells. In fact, the improperly severe response of the innate immune system is the key to conversion from a non-severe to severe disease state and needs to be investigated more deeply. The virus can also modulate the protective immune responses by developing immune evasion mechanisms, and thereby provide a more stable niche. Overall, combination of detrimental immunostimulatory and immunomodulatory properties of both the SARS-CoV-2 and immune cells does complicate the immune interplay. Thorough understanding of immunopathogenic basis of immune responses against SARS-CoV-2 has led to developing several advanced vaccines and immune-based therapeutics and should be expanded more rapidly. In this review, we tried to delineate the immunopathogenesis of SARS-CoV-2 in humans and to provide insight into more effective therapeutic and prophylactic strategies.