Background: Abatacept acts as a co-stimulation modulator preventing activation of T cells. Although it is approved for the treatment of rheumatoid arthritis (RA), its effects on adaptive immune response have not been fully elucidated. 

Objectives: To observe, in a cohort study, based on a clinical practice setting, the variation of peripheral blood T cells, immunoglobulin levels, and autoantibodies in the serum of RA patients during abatacept therapy. 

Methods: Our study comprised 48 RA patients treated with abatacept. All clinical data were collected at baseline and after 3 months of treatment. Clinical and laboratory tests included erythrocyte sedimentation rate, C-reactive protein, 28-joint disease activity score, RF, anti-citrullinated protein antibody, total immunoglobulins, immunoglobulin A (IgA), immunoglobulin G (IgG), immunoglobulin M (IgM), and lymphocyte sub-population. 

Results: Total immunoglobulin serum levels significantly decreased after 3 months of treatment and correlated positively with disease activity both at baseline and after 3 months of abatacept treatment. A reduction of serum IgM, IgG, IgA and RF was also demonstrated. The absolute number and percentage of cytotoxic (CD8+) T cells significantly decreased after 3 months of abatacept treatment, in particular the percentage of cytotoxic (CD8+) T cells significantly decreased only in patients responding to the treatment.

Conclusions: Our results highlight a different role of abatacept in the modulation of the adaptive immune response in RA by the reduction of polyclonal B-cell activation and cytotoxic T cells.