2096: Role of Cell-Bound Complement Fragments as Biomarkers to Determine Disease Activity in Patients with Systemic Lupus Erythematosus

Marcela Muñoz Urbano¹, Diana C. Quintero-González¹, Mauricio Rojas², Joaquín Rodelo¹, Alba Luz León Álvarez¹, Luis Gonzalez¹ and Gloria Vásquez³, ¹Universidad de Antioquia, Medellín, Colombia, ²Grupo de Inmunología Celular e Inmunogenética, GICIG, and Unidad de citometría de flujo, Sede de Investigación Universitaria, Universidad de Antioquia, Medellín, Colombia, ³Grupo de Inmunología Celular e Inmunogenética, GICIG and Grupo de Reumatología, Universidad de Antioquía, Medellín, Colombia

Background/Purpose: The prompt identification of patients with systemic lupus erythematosus (SLE) at risk of or in flare strongly influences the prognosis of the disease. Cell-bound complement activation products (CB-CAPs) have been studied as biomarkers of SLE activity. Despite growing knowledge, experience with CB-CAPs in the Latin American population is scarce. Therefore, this study aims to explore the behavior of CB-CAPs to determine lupus activity in SLE Colombian patients.

Methods: We conducted a cross-sectional study of 30 adult SLE patients (ACR/EULAR 2019 criteria) and 24 technical controls. Subjects were recruited from two centers in Medellín, Colombia, between September 2021 - January 2022. We used the non-serological SELENA/SLEDAI index (SELENA/SLEDAIns) to define activity as follows: inactive (0 points), mild/moderate (1-6 points), and severe (≥7 points) activity. We determined by flow cytometry the median fluorescence intensity (MFI) of the C3d and C4d bound to erythrocytes (E-C3d, E-C4d), reticulocytes (R-C3d, R-C4d), B lymphocytes (B-C3d, B-C4d), T CD4 (TCD4-C3d, TCD4-C4d), and T CD8 (TCD8-C3d, TCD8-C4d) lymphocytes. Descriptive correlations and an exploratory analysis of the accuracy of CB-CAPs to identify severe activity were performed.

Results: Table 1 shows the demographic and clinical characteristics of the SLE patients. Two inactive subjects, 11 with mild disease activity and 17 with severe activity, were included. Table 2 shows correlations between CB-CAPs with anti-dsDNA dilutions and serum complement. There was a statistically significant correlation between clinical SELENA-SLEDAIns with the TCD8-C4d MFI and TCD8-C3d MFI (figure 1). The results showed a negative correlation between C3 levels and SELENA-SLEDAIns for all patients (Rho= -0.57; p=˂0.001) and patients with severe disease activity (Rho= -0.65; p=0.004). In the case of C4, the correlation was -0.51 (p=0.004) for the 30 SLE subjects. Spearman correlation coefficients were not significant between anti-dsDNA by IIF and SELENA/SLEDAIns in none of the groups. The R-C4d had the highest sensitivity (75%), the best specificity (64%), and the area under the ROC curve (0.7 CI 95% 0.53-0.87). Therefore, R-C4d MFI was the CB-CAP with the best performance to discriminate lupus severe activity (SELENA/SLEDAIns ≥7 points).

Conclusion: This is the first time that TCD8-C4d and TCD8-C3d MFI demonstrated significant correlations with lupus disease activity. Previously, T-C4d was correlated with activity in an ethnical minority. To our knowledge, there are no data on this biomarker in large and diverse cohorts, which places a gap in the knowledge and an opportunity for future investigations. As we did, other authors have demonstrated the utility of R-C4d as a lupus disease activity biomarker.
APS, antiphospholipid syndrome; ELISA, enzyme-linked immunosorbent assay; IIF, indirect immunofluorescence; IQR, interquartile range; SD, standard deviation.
a. Taken at the time of recruitment.
b. Low complement was defined as either C3 or C4 levels below the normal range established at each of the laboratories.

C4d and C3d: complement fragments; E, erythrocytes; IIF, indirect immunofluorescence; MFI, median fluorescence intensity; TCD8, CD8+ T lymphocytes; R, reticulocytes.
a. Spearman's correlation coefficient was used to determine the correlation between CB-CAP levels with anti-dsDNA, C3, and C4.
b. C4d-/ C3d+ reticulocytes.
c. C4d+/ C3d- reticulocytes.
d. C4d+/ C3d+ erythrocytes.
e. C4d+/ C3d+ reticulocytes.

Figure 1. Correlations between CB-CAPs MFI with SELENA-SLEDAIns dilutions in the total SLE patient population. A shows the correlation between SELENA-SLEDAIns score and C4d MFI in CD8 T lymphocytes (TCD8-C4d MFI), and B represents the correlation between SELENA-SLEDAIns score and C3d MFI in CD8 T lymphocytes (TCD8-C3d MFI).
Disclosures: M. Muñoz Urbano, None; D. Quintero-González, None; M. Rojas, None; J. Rodelo, None; A. León Álvarez, None; L. Gonzalez, None; G. Vásquez, None.