Effect of substratum wettability and charge on adhesion in vitro and encapsulation in vivo by insect haemocytes.

The circulating leucocytes of insects, the haemocytes, adhere to and encapsulate foreign material that enters the insect's body cavity. The thickness of the capsule depends not only on the insect species but also on the nature of the object concerned, a fact that is of great importance to invading parasites and pathogens. In this paper, some of the factors that may stimulate haemocyte adhesion and subsequent encapsulation of the object have been investigated using abiotic materials with surfaces of different charge and wettability. The negativity and wettability of surfaces of polystyrene beads and plates can be increased by pretreatment with acid, and adhesion of haemocytes to these modified surfaces has been examined in vivo and in vitro. A similar proportion of haemocytes of the locust Schistocerca gregaria adhere to the plates in vitro, irrespective of the changes in charge and wettability, but the adhesion of haemocytes of the cockroach Periplaneta americana is proportional to the increases in both parameters. These differences in cell behaviour are reflected in vivo: cockroach haemocytes form thicker capsules around more hydrophilic and more negatively charged polystyrene beads, while locust cells encapsulate both types of surface to the same, minimal, degree. Positively and negatively charged Sepharose beads are encapsulated more thickly than are neutral beads in cockroaches; negatively charged Sepharose beads are not encapsulated at all in locusts. There are thus obvious differences between the two species in the ways in which their haemocytes respond to these modified abiotic surfaces. It is suggested that capsule thickness in vivo depends on the initial cell-substratum contact; different surfaces stimulate the cell to different extents, which in turn causes variations in the recruitment of other cells to the capsule.