After the advent of COVID-19, the rate of invasive candidiasis in SARS-CoV-2 infected patients has increased drastically and this has led to severe fatalities associated with co-infections (Pemán et al. 2020; Song et al. 2020). As the host immune response normally keeps development of systemic candidiasis at bay, thus it could be hypothesized that alterations in the host immune response during COVID-19 might be enhancing possibilities of this fungal co-infections. To proceed with our analysis, first we have extracted the upregulated genes in the blood of healthy humans which helps to combat C. albicans infections. Then, we have reconstructed PPINres(Fig. 1A) and determined module core genes (CGs) (Fig. 1B; Supplementary File 1). As CGs determine the function of PPINres rather than its centrality (Kovács et al. 2010), so functional enrichment of CGs with immune response related functions like- “cytokine response” (Fig. 1A) resembled that PPINresconstitute an intricate network of immunological signaling that confers resistance against Candida infections (Chin Kin et al. 2014; Griffiths et al. 2021). Notable CGs included -SRC, which has the ability to activate NFAT (nuclear factor of activated T cells) that aids T cell development (Kalia et al. 2021) and TRAF3 which elicits IL-17-induced signaling to prevent mucosal C. albicans infection (Zhu et al. 2010; Mengesha and Conti 2017). Thus, the CGs could be regarded as key players in the immunological signaling against C. albicans infections. Now, this immune response is being governed by interplay between TFs that regulate pathogen specific response. So, we reconstructed GRNres, to identify transcription factors (TFs) regulating the CGs (Fig. 2). Important TF-CG interactions include IRF1 and its interactor SLP1 which are required for CD8+ T cell differentiation (Zhang and Tsung 2017) and functioning of innate immune response (Williams et al. 2006) respectively. We also have detected a total of 7 network APs among which HES1, aids early T cell development in the thymus via the Notch signaling pathway(Seo and Taniuchi 2016). Since perturbation of APs might affect the network topology significantly (Tian et al. 2017), so subtle alterations in any of them might dampen the immune response against C. albicans infections to a greater amount.
Now, downregulation of these TF-CG interactions in GRNresduring COVID-19 could weaken host immune response and thus permit C. albicans co-infections. So, we initially identified downregulated genes in PBMCs of COVID-19 patients (Supplementary File 2) and mapped the immunomodulatory interactions of GRNreswith the downregulated gene pool of COVID-19. Among the TF-CG interactions (Fig. 3), notable interactions like ATF3-STX4 pair is found to be downregulated. Downregulation of such immunological mediators during COVID-19 could be altering host immune response because ATF3 was reported to regulate Th1 differentiation (Filén et al. 2010),while knockdown of STX4 reduces the cytotoxicity of human cytotoxic T cells (Spessott et al. 2017). Thus, we subsequently performed functional and pathway enrichment analysis of downregulated TF-CG pairs during COVID-19 (Fig. 4; Supplementary File 3). Our analysis showed that COVID-19 might disturb cell-mediated adaptive immune responseswhich could increase host’s susceptibility to Candida albicans infections.
Th17 and Treg subtypes particularly are indispensable for adaptive immunological response against candidiasis (Scheffold and Bacher 2020).Immunomodulatory signaling by IL-2R/STAT pathway, TGF-β/Smad and Notch pathway were evidenced to regulate Treg function for clearing of Candida albicans(Wang et al. 2022). To differentiate into Th17 subtype, TCRs activate IP3-DAG, MAPK and notch pathway to subsequently localize NF-κB (Krishna and Zhong 2013). IP3-DAG and MAPK pathway was also evidenced to activate AP-1 to mediate downstream signaling for T-cell differentiation. Activation of NF-κB and AP-1 mediated signalling triggers secretion of IL23, IL1β, TGFβ (Fujioka et al. 2004) which terminally diffrentiates naïve T-cells to Th17 cell type. We found in our analyses that both the subunits of AP-1 i.e. JUN and FOS, and NFKB1 are downregulated during COVID-19. Moreover, the chemokine receptor for Th17 i.e. CCR4(Acosta-Rodriguez et al. 2007) is also downregulated in PBMCs of COVID-19 patients (Fig. 5).
Tregs are also an important class of Th cells that regulate tissue specific diffrentiation of Th17 cells in candidiasis (Whibley and Gaffen 2014). Differentiation into this subtype requires stimulation of STAT5 by TCR (Welte et al. 1999). Activation of STAT5 activates FOXP3 which in turn forms a complex with RUNX1 to terminally differentiate naïve T-cells into Treg cells by secretion of IL2, IL15, IL16, IL-34and TGFβ(Zhang et al. 2008; Bézie et al. 2015; Apert et al. 2018; Hu et al. 2020). Moreover, RORγtcompetes with FOXP3 to bind with RUNX1 which if formed induces Th17 differentiation by releasing cytokines IL-17A, IL-17F, and IL-23(Ivanov et al. 2006; McGeachy and Cua 2008). Unusually, cytokines IL2, IL15, IL16, IL-34 and TGFβ for Treg differentiation and IL-17A, IL-17F, and IL-23A for Th17 differentiation are found to be downregulated during COVID-19 (Fig. 5). Not only the cytokines but other genes involved in these two T-cell differentiationsi.e., STAT5, FOXP3, RUNX1 and RORγt are also downregulated during COVID-19. Thus, weconcludethat Treg and Th17 differentiation pathways might be severely affected during COVID-19 which could make the patient palatable for Candida infections.
Our conclusions were further strengthened when we found that Notch signaling pathway, which alsohas the ability to decide fate of T cell differentiation to Treg and Th17 subtypes (Fig. 5)(Hosokawa and Rothenberg 2021), is affected due to COVID-19. T-cell differentiation by Notch signaling requires binding of JAG1 of APC to notch receptors Notch1/2 in T cell (Amsen et al. 2015).This activates GATA3 which in turn helps T-cell to differentiate into Tregby IL2 secretion(Wohlfert et al. 2011). Additionally, activation of NFκB and HES1 by notch signaling activates FOXP3 mediatedTreg function and development of Th17 cells by secreting IL17A respectively (Shin et al. 2014; Amsen et al. 2015; Ferrandino et al. 2018). Consequently, JAG1 ligand of APC along with GATA3 and HES1 from the notch signaling pathway were also found to be downregulated in COVID-19, Thus, disruption of IP3-DAG, MAPK and Notch signaling mediated Th17 and Treg differentiation pathway due to COVID-19 might aggravate chances of invasive candidiasis.
Additionally, we also noticed that expressions of APs in GRNres are suppressed during COVID-19. Strikingly, downregulated AP CEBPB was evidenced to be critical for pathogen clearance and host cell survival (Muñoz et al. 2019) in C. albicans infections. We also found NFKB1, HES1, CEBPB, CCR4 as downregulated APs which have already been shown in Fig. 5 to be involved in T-cell mediated adaptive immune response. Thus, during COVID-19, downregulation of these APs which exert important immunological functions to resist invasive candidiasis might be causing severity. Thus, experimental studies to elucidate their roles in conferring immunity against such co-infections should be conducted in future and also therapeutic procedures to optimize their expression should be designed to combat the disease.