Abstinent phagocytes after a burst of activity

Chronic autoimmune thrombocytopenic purpura (AITP) is an immune-mediated disorder in which platelets are opsonized by autoantibodies and prematurely destroyed by phagocytic cells in the reticuloendothelial system. Anti-D immunoglobulin that recognizes and binds specifically to the erythrocyte D antigen may be effective for some patients with idiopathic thrombocytopenic purpura (ITP). Its mechanisms of action are not fully understood. In this issue, Coopamah and colleagues (page 2862) report their novel studies showing that anti-D stimulates an Fc-dependent oxidative burst in the phagocytes. Subsequently, an inhibitory cytokine, interleukin 1 receptor antagonist (IL1ra), suppresses erythrophagocytosis by the phagocytes—in effect, feast followed by abstinence.

To study the effect of anti-D, the investigators developed flow cytometric assays to monitor various Fc-dependent events related to phagocytosis of opsonized red blood cells (RBCs). Anti-D–opsonized RBCs were able to induce a significant increase in reactive oxygen species (ROS) production in granulocytes and, to a lesser extent, in monocytes within a few minutes. In granulocytes, the generation of ROS was dose dependent on the amount of opsonized RBCs added to the assay. These results show that anti-D–opsonized RBCs enhance ROS production in both monocytes and granulocytes. The early ROS production may simply be a marker of Fc–Fc receptor (FcR) interaction at the leukocyte surface or, alternatively, may indicate that ROS acts as a second messenger.

After ROS production, erythrophagocytosis was significantly enhanced after 2 hours in the presence of anti-D, but by 6 hours this activity decreased. The decrease was not solely due to cell death because viability was unchanged. This suggests that phagocytosis was actively inhibited in vitro.

Based upon Coopmah et al's interest in cytokine changes in ITP with immunoglobulin therapy, they measured several cytokines over a 6-hour period of time within the phagocytes. Of all the cytokines tested only IL1ra was detected intracellularly and its increase correlated with a decline in phagocytosis. They hypothesized that IL1ra is responsible for inhibiting anti-D–opsonized RBC phagocytosis by leukocytes. Importantly, when ILra was added to their in vitro assays of phagocytosis they found a signifi-cant decrease in phagocytosis.

These series of experiments suggest that not only do anti-D–opsonized RBCs induce the production of IL1ra but that the cytokine can also actively inhibit the RBC phagocytosis. The exact mechanism of this inhibition is unclear at present. The IL1ra may act intracellularly or, alternatively, may inhibit intracellular signal transduction processes normally occurring after engagement of the receptor with IL1. Exploitation of these findings may lead to improved therapy of immune cytopenias, circumventing the need for plasma-derived blood products, such as anti-D, or high-dose immunoglobulin therapy.