Haemocytes and their Role in the Immune Response

Haemocytes are the immune cells of the insect and are found attached to internal organs of the insect, such as the fat body, trachea, or the digestive system, and circulating freely within the haemolymph. The density of haemocytes circulating in the haemolymph is indicative of infection, with low haemocyte densities being associated with infection and high haemocyte densities associated with healthy insects. This has been explicitly demonstrated by Bergin et al. (2003), where infection of G. mellonella larvae with the yeast Candida albicans resulted in greatly decreased numbers of circulating haemocytes, in contrast to larvae inoculated with the non-pathogenic yeast Saccharomyces cerevisiae, whose haemocyte decreased only slightly in number.

There are at least six types of haemocytes identified in lepidopteran insects (Figure 3.2) (e.g. G. mellonella). Here, the haemocyte classification of Price and Ratcliff (1974) is described. Haemocytes can be divided into the following types; prohaemocytes (6-13 |M), plasmatocytes (40-50 |M), granulocytes (45 |M), coagulocytes, sperulocytes (25 |M), and oenocytoids. Prohaemocytes are small round cells with large nuclei, which divide and are thought to differentiate into other cell types. Both plasmatocytes and granulocytes are involved in phagocytosis, nodule formation, and encapsulation (Tojo et al., 2000). Plasmatocytes are the most abundant haemocyte, have a leaf-like shape and contain lysosomal enzymes. Granulocytes have a small nucleus and a granule-rich cytoplasm. Spherules are not uniform in shape and have many small spherical inclusions. Oenocytoids are large, binucleate, non-phagtocytic cells, which may contain prophenoloxidase. Coagulocytes, as their name suggests, are involved in the clotting process. Haemocytes are also involved in the production of AMPs during the humoral response (Meister et al., 1994; Samakovlis et al., 1990).

Haemocytes with functions and characteristics similar to other vertebrate lymphocytes have been identified. Chernysh et al. (2004) reported haemocytes in Calliphora vicina with activities similar to those of mammalian cytotoxic lymphocytes. These haemocytes recognised the human myelogenous leukaemia K562 cells as non-self, attached to their surface and induced target destruction by inducing apoptosis of the K562 leukemia cells. Exposure of the K562 cells to these cytotoxic haemocytes caused a series of cytoplasmic-bulging movements termed zeiosis or membrane blebbing of the K562 cells. These effects were similar to effects observed of natural killer cells on K562 cells.

Haemocytes

Coagmocyre Adipohaemocyte Oenocytoid

Figure 3.2 Morphology of haemocytes involved in the cellular immune response. Possible differentiation of prohaemocytes into granulocyte, plasmatocytes and spherulocytes (marked by arrows). Not to scale

Coagmocyre Adipohaemocyte Oenocytoid

Figure 3.2 Morphology of haemocytes involved in the cellular immune response. Possible differentiation of prohaemocytes into granulocyte, plasmatocytes and spherulocytes (marked by arrows). Not to scale

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