Discussion:
Earlier studies have shown that in hospitalized patients, inflammatory biomarkers such as CRP, ferritin, LDH, d-dimer, and IL-6 can be used to predict clinical outcomes in COVID-19 patients[4-7]. The immune system plays a significant role in the disease’s clinical manifestation and progression, including the above-mentioned inflammatory markers. Therefore, the focus on immunological predictors that can be used early in the disease course to enable relocation of resources toward those at risk of getting the severe disease should be prioritized. In this study, we have shown that, in addition to elevated level of IL-6, the higher percentage of large inflammatory CD14+ CD16+ monocytes and lower percentage of naive CD27+CD28+ CD4+T cells are independent early immunological prognostic predictors of worse outcome in patients with COVID-19.
Similar to other existing data, in the present study, admitted patients were found to be older, diabetic, and hypertensive compared to those who did not require admission. Moreover, underlying heart diseases, chronic lung diseases, and chronic renal failure were noted more often in admitted patients than non-admitted patients, which is in agreement with previous studies [20-23]. In addition, current literature has ample information suggesting that high inflammatory markers can be used as a predictor of worse outcomes in admitted patients with COVID-19 infection. These included white blood cell count (WBC), absolute lymphocyte count, LDH, CRP, procalcitonin, D-dimer, ferritin, and ESR [20, 24]. This was also confirmed in our study, in which admitted patients had higher inflammatory markers including CRP, LDH, ferritin, fibrinogen, D-dimer, DIC score, and higher neutrophil/lymphocyte ratio.
The immune CD4+ and CD8+ T cells can be divided into four main subsets based on the surface expression of CCR7 and CD45RA. They reflect different maturation and T-cell differentiation stages that are functionally distinct. The subsets included naïve CD4+T cell subsets (CCR7+CD45RA+), central memory (TCM) CD4+T cells (CCR7+CD45RA-), effector memory CD4+T cells (TEM) and RA+ revertant effector memory (CCR-CD45RA+) (TEMRA)[25, 26]. Both CD4+ and CD8+ main four subsets can be further divided into different functional subsets based on the expression of CD27 and CD28 with different cytokine expression [12, 26, 27]. TEM that are CD27-CD28- are mainly IFN-g producers (Th1) compared to the CD27-CD28+TEM that are IL-2(Th0), INF-g(Th1), and IL-4 (Th2) producers[25]. TEM and TEMRA T cells are good cytokine producers, including IL-2, IFN-g, and TNF-a. Moreover, CD27-CD28- T cells have high effector capability similar to the terminal effector T cells TEMRA subset -[12, 26]. Similarly, the combination of PD-1 and CD57 can identify cells with exhausted and or cytotoxic phenotype[28, 29].
On the other hand, naïve T cell is mainly CD27+CD28+, and it is a good producer of IL-2 that is required for activation and proliferation [12, 25]. CD27+CD28+ naïve T cells are crucial in response to a new virus or vaccine. Those with reduced frequency of naïve T cells, as in the elderly, are at risk of getting a significant disease compared to those with plenty of naïve T cells that can respond better to such new viruses [30]. This was one of the important explanations for the increased mortality in the elderly after infection with SARS-CoV-2. This is in line with our findings that those with a low percentage of naïve CD4+T cells are at a higher risk of increased mortality.
De Biasi has compared the immune system in mild to moderate hospitalized patients(n=39), versus healthy uninfected group (n=25). This has shown a low count of total CD4+ and CD8+ T cells and their naïve and TCM subsets in the patient’s group. Moreover, these patients had a higher frequency of cells with senescent/exhausted phenotype (CD57+PD-1+) [15]. Similarly, we have found reduced percentage of naïve CD4+T and CD8+T and increased exhausted CD4+ and CD8+T cells in the hospitalized group early in the disease course. Studying patients with a milder illness almost three weeks from infection, non-admitted(n=17) compared to the admitted ones (n=13 moderate and n=9 severe), has shown a higher percentage of exhausted CD8+ T cells with a lesser cytotoxicity and inflammatory profile than the patients with severe manifestations [16]. We have, as well, showing that hospitalized group had increased percentage of exhausted and cytotoxic T-cell phenotype in the hospitalized group.
Monocytes are the key immune cells and good producers of inflammatory cytokines like IL-6 [31, 32]. They acquire a bigger size upon activation and viral infections [33], including severe COVID-19 [34]. Moreover, monocytes can be divided according to the differentiation stage using a combination of CD markers into (CD14+CD16-) immature, differentiated and inflammatory type (CD14+CD16+) and non-classical (CD14-CD16+) [35]. Examining monocytes in this way, has revealed higher percentage of large inflammatory monocytes CD14+ CD16+ detected in the hospitalized, in line with previous suggestions that patients with severe manifestation have bigger sized monocytes [33, 34, 36].