Nitric oxide differentially regulates T-cell function in rheumatoid arthritis and systemic lupus erythematosus

Experimental and clinical evidence for T-cell involvement in the pathogenesis of rheumatoid arthritis (RA) is compelling, and points to a local dysregulation of T-cell function in the inflamed joint. Nitric oxide (NO) has been shown to regulate T-cell function under physiological conditions, but overproduction of NO may contribute to lymphocyte dysfunction in RA. NO has recently been recognized as a key signaling intermediate for T-cell activation and mitochondrial biogenesis. NO is synthesized from L-arginine by NO synthetases (NOS). Three distinct isoforms of NOS are known, including neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS) enzymes. We previously detected the expression of eNOS and nNOS and the absence of iNOS in human PBL, while during inflammation macrophages and monocytes express iNOS. Systemic lupus erythematosus (SLE) is a systemic autoimmune disease of unknown origin characterized by the involvement of multiple organs. Several studies carried out on patients with both RA and SLE have documented increased endogenous NO synthesis, but its contribution to T-lymphocyte mitochondrial biogenesis and T-cell dysregulation is not known.

We investigated the role of NO in T-cell mitochondrial biogenesis in RA and SLE.

The mitochondrial mass, NO production and cytoplasmic Ca²⁺ levels were measured by flow cytometry. Mitochondria were visualized using transmission electron microscope.

T cells from RA patients produce >2.5 times more NO than T cells from healthy donors (P < 0.001). Unexpectedly, the mitochondrial mass was found to be similar in RA and control T cells (P = 0.65), whilst increased NO production was associated with increased cytoplasmic Ca²⁺ concentrations in RA T cells (P < 0.001). We observed that T-cell NO production decreased in most RA patients following anti-TNF treatment. Although lupus T cells produced comparable amounts of NO to normal T cells, lupus monocytes produced twice as much NO as normal monocytes (P = 0.015). We also observed increased mitochondrial mass (47.7 ± 2.8%; P = 0.00017) and increased cytoplasmic (38 ± 6.4%; P = 0.0023) Ca²⁺ content in T cells from SLE patients when compared with control donors. Electron microscopy revealed that T cells of lupus patients contained 8.76 ± 1 mitochondria, while control donors contained 3.18 ± 0.28 mitochondria per cell (P = 0.0009). In addition, lupus lymphocytes harbor several-fold enlarged mega-mitochondria. These data suggest that monocytes are the primary source of NO in SLE, while T lymphocytes are the primary source of NO in RA. Although the iNOS pathway is not as rapid as eNOS or nNOS, it is thought to be capable of generating much larger quantities of NO (nanomolar range) than the constitutive NOS isoforms (picomolar range), explaining the differences in T-cell mitochondrial biogenesis in SLE and RA. Since mitochondria can take up, store and release Ca²⁺, increased mitochondrial mass may account for altered Ca²⁺ handling in SLE. Furthermore increased NO production may contribute to T-cell dysfunction in both SLE and RA.