IL-6 is known to inhibit HLA-DR expression (Ohno et al., 2016), leading to the hypothesis that IL-6 over-production mediates the low HLA-DR expression on CD14 monocytes of COVID-19 patients. In agreement with this, negative correlation was found between serum amounts of IL-6 and the absolute number of HLA-DR molecules on CD14 monocytes of patients with COVID-19 but also between the absolute lymphocyte count and the absolute number of mHLA-DR on CD14 monocytes of patients with COVID-19 (Figures 5A and 5B). Furthermore, PBMCs from patients with immune dysregulation were cultured overnight in the presence of plasma of the COVID-19 patients, which was already shown to be rich in IL-6. The expression of HLA-DR on CD14 monocytes was strongly inhibited by COVID-19 plasma from patients with immune dysregulation but not by plasma from patients with an intermediate immune state of activation (Figures 5C–5F); the addition of the specific blocker of the IL-6 pathway Tocilizumab partially restored the expression of HLA-DR on monocytes of all patients with immune dysregulation (Figures 5E and 5F). Treatment with Tocilizumab in six patients was accompanied by increase of the absolute lymphocyte blood count within the first 24 h (Figure 5G). IL-6 was produced partly by CD14 monocytes and partly by CD4 cells (Figure 5H).
Figure 5 Immune Dysregulation Caused by SARS-CoV-2 Is Mediated by IL-6
(A) Negative correlation between serum amounts of IL-6 and the absolute numbers of the mHLA-DR on CD14 monocytes. The Spearman’s (rs) co-efficient of correlation and the respective p value are provided.
(B) Correlation between the absolute lymphocyte count and the absolute numbers of mHLA-DR on CD14 monocytes. The rs co-efficient of correlation and the respective p value are provided.
(C) Changes of the absolute numbers of mHLA-DR on CD14 monocytes of four patients infected by SARS-CoV-2 with intermediate state of immune activation after incubation with medium and their plasma.
(D) Changes of the MFI of HLA-DR on CD14 monocytes of four patients infected by SARS-CoV-2 with intermediate state of immune activation after incubation with medium and their plasma.
(E) Changes of the absolute numbers of mHLA-DR on CD14 monocytes of eight patients infected by SARS-CoV-2 with immune dysregulation after incubation with medium and their plasma; modulation by the addition of the specific IL-6 blocker Tocilizumab is also shown.
(F) Changes of the MFI of HLA-DR on CD14 monocytes of eight patients infected by SARS-CoV-2 with immune dysregulation after incubation with medium and their plasma; modulation by the addition of the specific IL-6 blocker tocilizumab is also shown.
(G) Changes of the absolute lymphocyte count of six patients before and after start of treatment with Tocilizumab.
(H) Intracellular staining for IL-6 in CD14 monocytes and in CD4 lymphocytes of three patients infected by SARS-CoV-2 with immune dysregulation.
Statistical comparisons are indicated by the arrows; ns: non-significant; ∗p < 0.05; ∗∗p < 0.01.