Cellular and morphological changes in lymphoid organs after a single injection of interleukin 1 alpha in the mouse

We investigated the effects of a single i.v. injection of recombinant human interleukin 1 alpha (IL-1 alpha) on the morphology and the cellularity of several lymphoid organs in normal mice. The injection of 100 U of IL-1 alpha resulted in maximal neutrophilia and leukocytosis at 1 h. By 72 h, the numbers of mononuclears, but not that of polymorphonuclears, returned to baseline levels. Absolute increase in mononuclears was paralleled by relative lymphopenia. Changes in the peripheral blood coincided with rapid decrease in the spleen cellularity and white pulp volume (especially the marginal zone), and an increase in the red pulp volume. Bone marrow cellularity was increased at 1 h, but returned to control levels by 6 h after IL-1 injection. Thymus cell depletion and cortex atrophy were maximal at 6 h and could be observed throughout the experiment. These findings indicate that leukocytosis induced by a single i.v. injection of IL-1 alpha in normal mice is concomitant with a rapid cell depletion of the spleen and thymus. Morphological and cellular changes in lymphoid organs may represent the mobilization of immunocompetent cells during the development of the inflammatory response.