The SARS-CoV-2 virus enters the body via mouth, nose, or eyes then travels to the nasal area and the mucous membranes of throat, where it attaches to cells with high expression of ACE2 receptors through its spike proteins. Afterward, it starts to multiply and eventually moves into the lungs [1]. Throughout the pandemic, the virus underwent numerous mutations, leading to the emergence of dominant strains that had a significant impact on people's health worldwide. The primary variants of Covid-19, including alpha, beta, gamma, delta, and omicron, have significantly impacted global infection rates [2]. However, an analysis of Covid-19 nucleotide sequences deposited on the Global Initiative on Sharing All Influenza Database (GISAID) from India reveals that alpha (B.1.1.7) and delta (B1.617.2) variants dominated during the second wave (April 2021 to June 2021), while omicron (B.1.1.529) and its sub-variant (BA.2.75) gained prominence in the third wave (December 2021 to January 2022) [3]. The alpha and delta variants exhibited distinctive Receptor Binding Domain (RBD) mutations, including N501Y and E484K, and L452R and T478K, respectively. In contrast, the Omicron variant displayed approximately 15 mutations in its RBD [4]. Despite these mutations in Covid-19 variants, the ACE2 receptor remains the major point of entry for the virus and its variants, indicating a similar host immune response and symptoms regardless of variants, age and gender [5].

ACE2 is abundantly present in the lungs, heart, and kidneys, and it is released into the bloodstream, where it plays a crucial role in controlling various molecular pathways [6]. The SARS-CoV-2 virus disrupts the balance between ACE and ACE2, contributing significantly to the severity of COVID-19, particularly in individuals with co-morbidities [7]. This disturbance in the ACE/ACE2 equilibrium also disrupts the function of downstream signaling molecules, including components of the host immune system such as Toll-like receptors (TLRs), T-cells, B-cells, and the balance of TH1/TH2 cytokines. This disruption leads to inflammation and the potential for a cytokine storm [8].

Toll-like receptors (TLRs) identify viral particles that have been processed and presented by macrophages, leading to the activation of the innate immune system [9]. The activated TLR pathway results in the production of pro-inflammatory cytokines such as IL-6, IL-1, TNF-α, and type 1 interferons [10]. IL-6, in particular, plays a pivotal role in modulating the immune response, promoting the proliferation and differentiation of CD8 cells, T-cells, and B-cells through the activation of RANKL. Additionally, it stimulates antibody production, further contributing to the cytokine storm, which is induced by IL-1 [11]. Notably, IL-1 cytokine plays a crucial role in triggering the cytokine storm by disrupting the balanced immune responses caused by COVID-19 infection [12].

The hyperactive immune responses triggered by SARS-CoV-2 infection are likely the primary cause of severe pneumonia, acute lung injury, which can progress to ARDS syndrome, and ultimately result in multiple organ failure and death. During a COVID-19 infection, the virus releases its proteins, including PLpro (papain-like protease), into the host. This protein plays a significant role in the antiviral immune response and disrupts the immune system by deactivating the TLR pathway [13]. PLpro possesses the capability to ubiquitinate crucial host-cell proteins involved in key pathways, aiding the COVID-19 virus in evading the host's immune defenses.

The clinical differences observed among individuals infected with the SARS-CoV-2 virus may be attributed to either the virus's ability to invade or evade the immune response, as well as other crucial factors such as virus mutations, virulence, genetic background, co-morbidities, age, sex, and lifestyle. Therefore, gaining insights into why some individuals remain asymptomatic while others develop severe or critical cases, potentially leading to death, could provide a more effective approach to managing the severity of COVID-19.

In this study, our primary focus is to address the question of how COVID-19 affects ACE2 expression and the subsequent cascade of signaling events, including the expression of IL-6, IL-1, and RANKL. We aim to explore how these factors are associated with various symptoms and responses in individuals based on their age and gender. This research aims to uncover how the COVID-19 virus interacts with the host's immune system, either by evading it or invading it, particularly through the viral protein PLpro. By highlighting key biomarkers, this study may offer insights into potential indicators of severe or critical conditions during COVID-19 infection.