Influence of encapsulation on phagocytosis of Pasteurella multocida by bovine neutrophils

The effect of encapsulation on phagocytosis of Pasteurella multocida by bovine neutrophils was examined by using two encapsulated strains, NA77 (capsular type A) and C42 (capsular type B), and comparing them with an unencapsulated counterpart strain, 1173. The uptake of [(3)H]thymidine-labeled bacteria by neutrophils was quantitatively measured after incubation of the bacteria in normal bovine serum, heat-inactivated serum, and hyperimmune sera (anti-NA77 and anti-C42). Results showed that all three strains of P. multocida were ineffectively opsonized by the heat-labile serum complement system. The unencapsulated strain was completely phagocytized in the presence of heat-stable opsonins found in normal serum. Although encapsulation of strain C42 was found to interfere with opsonization by normal serum, this strain was completely phagocytized when hyperimmune serum (anti-C42) was used as the opsonin source. These results suggest that specific anticapsular antibodies found in the hyperimmune serum readily opsonized the encapsulated strain C42 and enhanced phagocytosis. The presence of a thick capsule on strain NA77 interfered with phagocytosis in the presence of normal or hyperimmune serum (anti-NA77). This interference was due to the presence of hyaluronic acid which was a major component of the capsule. Treatment of this encapsulated strain with hyaluronidase decapsulated the bacteria. Bacteria treated in this way were almost completely phagocytized (90%) in the presence of heat-stable opsonins. The exact mechanism by which the capsule of P. multocida NA77 interfered with phagocytosis was not demonstrated; perhaps the slimy nature of the hyaluronic acid makes the phagocytic act difficult by changing the physiochemical surface properties, or it may prevent opsonization.