Mechanisms of ANCA-mediated leukocyte-endothelial cell interactions in vivo

Anti-myeloperoxidase (anti-MPO) antibodies have been implicated in the pathogenesis of small-vessel vasculitis, but the molecular mechanisms by which these antibodies contribute to disease are unknown. For determination of how anti-MPO antibodies affect inflammatory cell recruitment in small-vessel vasculitis, intravital microscopy was used to monitor leukocyte behavior in the accessible cremasteric microvessels under various experimental conditions. After local pretreatment of the cremaster muscle with cytokines (TNF-alpha, IL-1beta, or keratinocyte-derived chemokine), administration of anti-MPO IgG to wild-type mice reduced leukocyte rolling in favor of augmented adhesion to and transmigration across the endothelium. This led to a decrease in the number of systemic circulating leukocytes and, similar to the early events in the development of vasculitic lesions, an increase in leukocyte recruitment to renal and pulmonary tissue. TNF-alpha led to the greatest recruitment of inflammatory cells, and IL-1beta led to the least. When anti-CD18 was co-administered, anti-MPO IgG did not affect leukocyte rolling, adhesion, or transmigration; similarly, anti-MPO IgG did not produce these effects in Fc receptor gamma chain-/- mice. This study provides direct in vivo evidence of enhanced leukocyte-endothelial cell interactions in the presence of anti-MPO IgG and highlights the critical roles of Fcgamma receptors and beta2 integrins in mediating these interactions. In addition, it suggests that neutrophils primed by cytokines in the presence of anti-MPO IgG can have systemic effects and target specific vascular beds.