Immunodynamics of methylprednisolone induced T-cell trafficking and deactivation using whole blood lymphocyte proliferation techniques in the rat

Glucocorticoids have diverse effects on various components of the immune system and assessment of such activities in vivo often involves complex techniques and numerous animals. We developed a whole blood technique for determining proliferation rate of lymphocytes in minute amounts of rat blood (5 μL as opposed to a whole rat spleen). This method was used in assessment of in vivo T-cell deactivation by methylprednisolone (MP). The blockade of this process by the anti-glucocorticoid, RU 40555, also allows measurement of T-lymphocyte trafficking between vascular and extravascular pools. Blood samples were taken over several hours after iv MP administration to adrenalectomized rats, MP concentrations and lympho-proliferative activities were determined ex vivo after mitogen activation with and without blocking MP with RU 40555. MP disposition was mono-exponential with a t(1/2) of 34 min. The pharmacodynamics (PD) of T-cell trafficking was modeled with a physiological indirect model to generate the IC₅₀ (0.4 ng/mL) for the inhibitory action of MP on return of T-cells to blood as well as cell trafficking rate constants. The overall suppression of blood T-cells was modeled with an equation which accounts directly for inhibition of the proliferation activity of available blood T-cells with an DC₅₀ of 0.37 ng/mL. MP produced an initial influx of T-cells to blood within 1 h of infusion, a later marked T-cell depletion with a nadir at 4 h, and return to baseline by 9 h. Lymphocyte deactivation occurred within minutes of MP infusion and returned to baseline in 9 h. MP action was prolonged owing to the low IC₅₀. This approach for assessing dual features of corticosteroid effects on T-cell trafficking and deactivation allows quantitative PK/PD modeling in small animals such as the rat.