Background Granzymes are a family of serine proteases which play key roles in the induction of target cell death by cytotoxic T-lymphocytes and NK cells. In particular, granzyme B (GZMB) is expressed by activated CD4+ and CD8+ T-cells, while it less abundant in NK cells (1). Soluble GZMB levels are elevated in the sera of patients with rheumatoid arthritis (RA), and are associated with early development of radiographic erosions. The possible role of GZMB in the pathogenesis of joint damage was suggested by in vitro data demonstrating its enzymatic activity for the cleavage of cartilage aggrecan proteoglycans and by the associations of a genetic variant of GZMB with progression of joint destruction (2). RA is characterized by several changes of the T-cell compartment which include, in most patients, an increased number of T-cells lacking the CD28 costimulatory molecule, which display some functional characteristic of cytotoxic memory T-cells.
Objectives Since CD28 negative (CD28-) T-cells may be an important source of GZMB, and we have previously shown that co-stimulation blockade by abatacept can prevent the generation of the CD28- T-cell populations (3), we evaluated the effect of abatacept therapy on GZMB serum levels in patients with RA.
Methods Fifty-three RA patients, treated for at least 3 months with abatacept were evaluated. Disease activity and response to the treatment were measured with DAS28 (based on CRP) and EULAR Criteria. T-cell counts were determined by flow cytometry. Serum GZMB samples were collected before the first administration of abatacept (T0) and then after 6 months (T6), and measured by an indirect solid-phase enzyme immunoassay with a sensitivity limit of 20 pg/ml.
Results The percentage and the absolute number of circulating CD4+CD28- T-cells decreased after ABA therapy (T0 vs. T6: p=0.018; p=0.018, respectively), as well as those of CD8+CD28- T-cells (p=0.005; p=0.008). At T0, GZMB serum levels were correlated with disease activity (p=0.002) and percentages of circulating CD4+CD28- and CD8+CD28- T-cells (p=0.007; p=0.031). In 25 patients serum levels of GZMB were evaluated also at T6: in 18 patients with a moderate or good clinical response to abatacept the levels of GZMB significantly decreased from T0 (median: 62.8 pg/ml [10th-90th percentile: 45.8-116]) to T6 (53.8 pg/ml [46.4-96.6]; p=0.023), whereas no variation was observed in 7 non responders. The variation of GZMB levels was directly correlated with that of DAS28-CRP (p=0.040), but not with those of circulating CD28- T-cell subsets.
Conclusions Costimulation blockade by abatacept can decrease the serum levels of GZMB in RA patients responding to the treatment, suggesting that this might be one of the mechanism by which abatacept can prevent radiographic erosions. However, the lack of correlation of such decrease with the numbers of circulating CD28- T-cells suggests that these cells are not the main source of serum GZMB.
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Acknowledgements Bristol-Myers-Squibb Italy provided an unrestricted research grant for the study conduct and did not interfere with the conception and design of the study, acquisition, analysis, interpretation of data, and manuscript drafting. The authors declare that they have no other competing interests.
Disclosure of Interest None declared