BAFF-var as a Biomarker of Response to B-depletive Treatment in Systemic Lupus Erythematosus and Rheumatoid Arthritis

BACKGROUND

Systemic lupus erythematosus (SLE) and Rheumatoid Arthritis (RA) are multisystem immune-mediated disease characterized by a complex pathogenesis, where an abnormal activation of the humoral immune response, with overproduction of autoantibodies, demonstrated to have a pivotal role.

B lymphocyte stimulator (BAFF) is a cytokine and drug target that is primarily produced by monocytes and neutrophils. It is essential for B-cell activation, differentiation and survival. Overexpression of BAFF was demonstrated to be associated with more severe pattern of SLE and correlate to more frequent positivity for anti-Sm (OR 1.7; 95% CI 1.3, 2.2), high titre anti-dsDNA (OR 1.5; 95% CI 1.1, 2.2), lower C3 (OR 1.3; 95% CI 1.0, 1.8), higher proteinuria (OR 1.8; 95% CI 1.4, 2.4) and increased risk of flare (HR 1.86; 95% CI 1.29, 2.68). Similarly, high levels of BAFF have been observed in blood and synovial fluid of RA patients and were demonstrated to correlate with higher disease activity in RA.

Belimumab and rituximab are the prototypical form of B-cell targeted therapies currently approved for SLE and RA, respectively.

Belimumab, a fully humanized monoclonal antibody directed against BAFF, is the first biological drug licensed and approved for use in combination with standard immunosuppressants in SLE. Data from RCTs demonstrated that higher disease activity, anti-dsDNA positivity, low complement and corticosteroid treatment may predict a higher benefit to belimumab. However, around 40% of subjects did not achieve an adequate response.

Rituximab (RTX) is a chimeric monoclonal antibody that of selectively inhibit the CD20 receptor on the surface of B lymphocytes. The use of RTX, preferably in combination with methotrexate, is indicated in patients with RA unresponsive to treatment with traditional immunosuppressants. Although greater efficacy was recorded in patients with positivity for autoantibodies (e.g. rheumatoid factor and anti-citrullinated peptide), also in this case a about 30-40% of patients is inadequately responsive.

A variant of TNFSF13B gene encoding for BAFF (BAFF-var), especially common across Sardinia (allele frequency, 26.5%) and progressively less common in southern (5.7% in Italy, 5.0% in Spain and Portugal) and northern Europe (1.8% in the UK and Sweden), was associated with autoimmune diseases, including SLE and RA. The causal variant was identified as an insertion-deletion variant, yielding a shorter transcript that escaped microRNA inhibition and increased production of soluble BAFF, which in turn up-regulated humoral immunity. In particular, BAFF-var was associated with increased level of soluble BAFF (effect size 0.77 standard deviations (SD); P= 8.47 x10-150), IgG (0.31 SD, p=1.68x10-12), IgA (0.31 SD, p=7.64x10-9), IgM (0.31 SD, p=4.70x10-8), as well as increased number of total B cell (0.18 SD, P=4.23x10-12) and several subtypes, including the CD24+CD27+ (comprising un-switched and switched memory) (0.22 SD, P=2.94x10-10) as the strongly associated. Further, transcriptome data demonstrated BAFF-var as an expression quantitative trait locus that increases TNFSF13B expression (P = 2.37×10-13). However, the elevated mRNA levels could explain only 24-27% of the effect on protein, indicating that an increase in translation level was also probably involved. In this regard, data indicate that BAFF-var raises soluble BAFF levels by favoring the production of a shorter transcript that is less inhibited by microRNA, such as miR-15a6.

Knowing the effective influence of BAFF-var in the immunological, molecular and clinical modifications induced by BAFF-inhibition has a crucial importance In the prospective of a personalized medicine in SLE and RA patients.

OBJETVIES

• Primary objective: To explore the immunological basis of a potential role of BAFF-var as a prognostic biomarker for response to belimumab or rituximab in the treatment SLE and RA patients, respectively. More in detail, the study aims to evaluate if the condition of BAFF-var carrier influences the drug-related modifications of immunological and molecular variables, such as: (a) soluble BAFF (BAFFs) cytokine, (b) mRNA-BAFF (c) miRNA-15a (d) B-cell subpopulations.
• Secondary objectives: To explore the effect of BAFF-var on the clinical response rate to belimumab or rituximab, as assessed by different outcome measures.

RESEARCH DESIGN AND METHODOLOGY

• Study design: The proposed project is an observational longitudinal study
• Population: Distribution of BAFF-var and modifications on immunological, molecular and clinical variables, after starting treatment with anti-BAFF agent, will be analyzed on a real-life monocentric Sardinian cohort of SLE and RA patients. In this regard, the Sardinian represents the most suitable population for this kind of analysis because of the higher frequency of BAFF-var; indeed the condition of BAFF-var carrier is expected in 50-60% of patients. About 30 consecutive SLE and 30 consecutive RA patents referred to the Rheumatology Unit at the Department of Medical Sciences and Public Heath of the University and University Clinic of Cagliari will be recruited.
• Data collection: Demographic (e.g. gender, age at enrolment, age of onset and diagnosis), clinical (e.g. cumulative and active disease manifestations, clinimetric measures), serological (e.g. autoantibodies, including anti-dsDNA, C3 and C4, IgG, IgA, IgM, RF, ACPA) and therapeutic (e.g past and current therapies) will be collected.
• BAFF genotyping: For BAFF genotyping, whole blood samples will be collected in EDTA at the enrollment. The gDNA will be extracted by the salting-out method according to standard protocol. Customs TaqMan assay (Thermo Fisher Scientific) specifically designed to genotype the BAFF variants of interest will be used with allele-specific Real Time PCR in all samples.
• B-cell phenotyping: Flow cytometry will be used to profile B-cell subpopulations in the 30 enrolled SLE patients. The number of B cell (CD45+CD19+), as well as the absolute and relative number of B transitional (CD24+CD38+), antibody secreting (ASC, CD27+CD38+), B memory switched (CD27+IgD-), B memory un-switched (CD27+IgD+), B naïve (IgD+CD27-CD24-) and B regulatory (CD24+CD27+) cells will be assessed by Lyse-No-Wash protocol using BD TruCountTM (Becton Dickinson).
• Soluble BAFF: it will be measured by enzyme-linked immunosorbent assay (ELISA)
• Gene expression study: mRNA will be extracted from whole blood by Trizol Reagent and the quantitative analysis of mRNA-BAFF and miRNA-15a will be performed by Real-Time PCR using specific TaqMan probes, after reverse transcription in cDNA.
• Statistical analysis. Statistical analysis will be performed by using the SPSS® statistical software. Comparison of mean levels of sBAFF, mRNA-BAFF, miRNA-15a and different B-cell phenotypes between BAFF-var + and BAFF-var - patients at each visit will be performed by T- Student test or Mann Whitney test, if appropriate. Comparison of the mean difference of levels of sBAFF, mRNA-BAFF, miRNA-15a and B-cell subpopulations between BAFF-var + and BAFF-var - patients at the different visits intervals will be performed by T- Student test or Mann Whitney test, if appropriate. The effect of BAFF-var on modification of the levels of the immunological and molecular variables above will be further assessed by multiple regression, including as independent variables, concomitant treatments and baseline disease features. Fisher's exact test and logistic regression analysis will be performed to compare the response rate, as assessed by different efficacy measures, in BAFF-var + and BAFF-var - patients.