Canine cutaneous and systemic histiocytosis - Reactive histiocytosis of dermal dendritic cells

Canine histiocytic proliferative disorders include reactive diseases such a s cutaneous and systemic histiocytosis and neoplastic diseases such as cuta neous histiocytoma and localized and disseminated histiocytic sarcoma (mali gnant histiocytosis). Their etiology and pathogenesis are unknown. Canine c utaneous and systemic histiocytosis target the skin and subcutis and have s imilar clinical behavior. Systemic histiocytosis also affects other organ s ystems. Clinicopathologic and phenotypic features of canine cutaneous and s ystemic histiocytosis were examined in this study. Canine cutaneous (18 cas es) and systemic (26 cases) histiocytosis were characterized by angiocentri c, pleocellular accumulations consisting of CD1(+), CD11c(+), MHC II+, CD4( +), and Thy-1(+) (CD90) activated dermal dendritic antigen-presenting cells (APC) with admired CD3(+), CD8(+), TCR alpha beta(+) T lymphocytes, and ne utrophils. Hence, canine cutaneous and systemic histiocytosis represent two clinical manifestations of a reactive proliferation of dermal dendritic ce lls. Cultures and special stains failed to identify infectious agents. Cani ne reactive histiocytoses respond to immunosuppressive therapy (cyclosporin e A or leflunomide). Therefore, immunedysregulatory mechanisms are likely t o be involved. Spontaneous reactive histiocytoses are frequently seen in do gs, and they constitute an excellent model to study pathologic mechanisms i nvolved in reactive proliferations of dermal dendritic APC.