Levels of soluble complement regulators predict severity of COVID-19 symptoms

Abstract

The SARS-CoV-2 virus continues to cause significant morbidity and mortality worldwide from COVID-19. One of the major challenges of patient management is the broad range of symptoms observed. While the majority of individuals experience relatively mild disease, a significant minority of patients require hospitalisation, with COVID-19 still proving fatal for some. As such, there remains a desperate need to better understand what drives this severe disease, both in terms of the underlying biology, but also to potentially predict at diagnosis which patients are likely to require further interventions, thus enabling better outcomes for both patients and healthcare systems. Several lines of evidence have pointed to dysregulation of the complement cascade as a major factor in severe COVID-19 outcomes. How this is underpinned mechanistically is not known. Here, we have focussed on the role of the soluble complement regulators Complement Factor H (FH), its splice variant Factor H-like 1 (FHL-1) and five Factor H-Related proteins (FHR1-5). Using a targeted mass spectrometry approach, we quantified these proteins in a cohort of 188 plasma samples from controls and SARS-CoV-2 patients taken at diagnosis. This analysis revealed significant elevations in all FHR proteins, but not FH, in patients with more severe disease, particularly FHR2 and FHR5 (FHR2: 1.97-fold, p<0.0001; FHR5: 2.4-fold, p<0.0001). Furthermore, for a subset of 77 SARS-CoV-2 +ve patients we also analysed time course samples taken approximately 28 days post-diagnosis. Here, we see complement regulator levels drop in all individuals with asymptomatic or mild disease, but regulators remain high in those with more severe outcomes, with elevations in FHR2 over baseline levels in this group. These data support the hypothesis that elevation of circulating levels of the FHR family of proteins could predict disease severity in COVID-19 patients, and that the duration of elevation (or lack of immune activation resolution) may be partly responsible for driving poor outcomes in COVID-19.

Keywords: COVID-19; SARS-CoV-2; biomarkers; complement; factor H; factor H-related proteins; mass spectrometry.

Figure 1

Circulating concentration of FHL-1 and FHR1 to FHR5 is elevated in COVID-19 patients. Scatter plots of FH (A), FHL1 (B), and FHR1 to FHR5 (C–G) represent the measured plasma protein concentration in nM. Pairwise adjusted p-values were obtained from a Tukey’s multiple comparisons test (4 d.p.).

Figure 2

Circulating complement cofactors predict disease severity. Receiver Operating Characteristic (ROC) curves for control versus severe [group (E)] disease are shown for circulating FHL-1 (A) and FHRs 1-5 (B-F). Area under the curve is provided to 3 d.p.

Figure 3

Circulating concentrations of FHL-1 and FHR1 to FHR5 in COVID-19 patients after 28 days. Scatter plots of FH (A), FHL1 (B), and FHR1 to FHR5 (C–G) represent the measured plasma protein concentration in nM. Pairwise adjusted p-values were obtained from a Tukey’s multiple comparisons test (4 d.p.).

Figure 4

Selected pairwise comparisons of circulating concentrations of FHL-1 and FHR1 to FHR5 in COVID-19 patients after 28 days. (A) CFH, FHL-1 and FHR5 in asymptomatic to moderate symptomatic individuals. (B) CFH, FHL-1 and FHR1-5 in patients with critical (Group E) COVID-19.