Activated T cells play a central role in the inflammatory cascade leading to the joint inflammation and destruction characteristic of rheumatoid arthritis (RA). The cytokines secreted by activated T cells are thought to both initiate and propagate the immunologically driven inflammation associated with RA.

Abatacept, the first of a new class of agents for the treatment of RA that selectively modulates the co-stimulatory signal required for full T-cell activation, was evaluated for its ability to regulate human T-cell proliferation and cytokine production in vitro. The effect of abatacept on lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α) from monocytes was evaluated to distinguish the impact of this agent on innate versus adaptive, antigen-specific immune responses.

T cells were isolated from normal healthy human volunteers. The effect of abatacept on antigen-dependent T-cell activation was evaluated using either an irradiated human B-cell line (PM-LCL) as the antigen-presenting cells (APCs) for a primary mixed lymphocyte reaction (MLR), or autologous E-PBMCs as APCs for a recall response to tetanus toxin (TT). Cytokines were measured at various times post activation, with proliferation determined on day 5. Monocytes were isolated by elutriation, challenged with LPS and TNF-α levels measured at 6 hours. Chi L6 was included as a non-specific fusion protein control.

Abatacept significantly downmodulated T-cell proliferation, in both primary and recall responses, at concentrations between 0.3 and 100 μg/ml, with maximal inhibition (~60–80%) observed at ~3–10 μg/ml. These concentrations are below the abatacept trough plasma levels observed in patients receiving a clinically effective dose 1. Under conditions of maximal inhibition of proliferation, and similar to trough plasma levels in patients (30 μg/ml), abatacept also inhibited IL-2, TNF-α and interferon gamma secretion in both primary and TT-dependent recall responses. However, the extent, kinetics and rank order of cytokine inhibition by abatacept was somewhat different between primary and recall responses. In contrast, abatacept did not inhibit LPS-induced TNF-α production in primary human monocytes, demonstrating that its action is restricted to antigen-dependent T-cell responses.

Abatacept, a selective co-stimulation modulator, significantly inhibited the activation (as measured by cytokine production) and proliferation of human T cells in the context of a primary MLR or TT-dependent memory response. This inhibition occurred at concentrations below the serum C_min levels observed in patients receiving a clinically effective dose of abatacept 1 (10 mg/kg monthly), consistent with suppression of T-cell activation in vivo. There was no effect of abatacept on LPS-stimulated TNF-α production in monocytes indicating that this agent may largely preserve innate immune responses.