THE HEMATOLOGY OF TRYPANOSOMA-CONGOLENSE INFECTION IN CATTLE .2. MACROPHAGE STRUCTURE AND FUNCTION IN THE BONE-MARROW OF BORAN CATTLE

Macrophages (M phi) in smears and sections of sternal bone marrow (BM) derived by weekly sequential biopsies from five adult Boran cattle re -challenged with Trypanosoma congolense were studied by light and tran smission electron microscopy (TEM). Cells of the mononuclear phagocyte system including monoblasts, promonocytes, monocytes and M phi increa sed severalfold in the sinusoids and haemopoietic compartment (HC) of the BM during infection. M phi activation occurred with significant in creases (p<0.001) in M phi size and numbers of organelles including mi tochondria, lysosomes and rough endoplasmic reticulum. Light microscop ic examination of the BM smears showed that 25.8% of 1200 M phi examin ed phagocytosed many non-mitotic haemopoietic cells of the erythroid a nd granulocytic series as well as mature erythrocytes and thrombocytes but seldom lymphocytes from day 29 postinfection (dpi), when the firs t peak of parasitaemia occurred, until and termination of the experime nt on 98 dpi. Some of the Mer with phagocytosed cells (10.4%) had cell s from more than one lineage. TEM confirmed cytophagia and showed that the process begins with cell to M phi attraction characterised by dev elopment of microvilli at the surface of contact by the target cell an d of enveloping pseudopodia:by the Mel. This was followed by target ce ll. to M phi adhesion and finally phagocytosis. The cells being phagoc ytosed and those freshly engulfed appeared morphologically normal. Man y Mer were heavily laden with haemosiderin in the chronic phase of the infection (78 and 98 dpi). TEM showed that the activated M phi in the BM developed extensive contacts through reciprocal blunt microvilli w ith the haemopoietic cells. Macrophages were absent from the sinusoids of the BM prior to infection but became numerous during infection, an d were adhered to sinusoidal endothelial cells by reciprocal blunt mic rovilli. These M phi phagocytosed blood cells (erythrocytes, neutrophi ls, thrombocytes), and free trypanosomes which, though present in the arterioles of the BM, were never seen in the sinusoids and HC of the B M. This study indicates that the Mpi plays very vital roles in regulat ing and executing the events in the BM during T. congolense infection of cattle.