Our previous studies demonstrated that exposure of CD4⁺ enriched T lymphocytes to an IL-4 antagonist significantly upregulates IL-10 secretion. IL-10 is produced by Th1- and Th2-like T effector lymphocytes but the primary T cell source is T regulatory cells. In addition, IL-10 is secreted as part of a homeostatic anti-inflammatory mechanism during cellular apoptosis. These studies were performed to identify the T cell source of IL-10.

Enriched (>95% pure) resting, naïve CD4⁺ T lymphocytes were obtained by negative selection using magnetic beads (Miltenyl). Cells were left in a resting state or exposed to IL-4 antagonism using either soluble IL-4 receptors (sIL-4R) or anti-IL-4 receptor antibody. After 4 days, cells were evaluated for IL-10 production, phenotypic markers for T regulatory cells (CD25⁺, CD45rb^lo), and apoptosis (propridium iodide and annexin V expression).

IL-4 antagonism upregulated IL-10 secretion (0.5 ± 0.4 pg/ml to 7.2 ± 3.0 pg/ml; p<.03). These effects were more pronounced with the anti-receptor antibody than with sIL-4R. Secretion of IL-10 was associated with increased expression of apoptotic (annexin V⁺) cells (28.4 ± 3.2% to 35.1 ± 11.6%). IL-4 antagonism inhibited expression of the anti-apoptotic protein bcl-2.

NaïVe T cells undergo apoptosis if not differentiated into effector T cells by foreign antigen or protected by low affinity engagement of their T cell receptor by MHC/self antigen. Apoptosis is delayed by the autocrine effects of constitutively produced IL-4. Agents which block IL-4 in the immune synapse (anti-IL4R) as opposed to secreted distally-acting forms of IL-4 (sIL-4R) accelerate the development of apoptosis and thereby promote IL-10 secretion.