Cellular immune responses require the generation and recruitment of diverse blood
cell types that recognize and kill pathogens. In Drosophila melanogaster larvae, immune-inducible
lamellocytes participate in recognizing and killing parasitoid wasp eggs. However,
the sequence of events required for lamellocyte generation remains controversial.
To study the cellular immune system, we developed a flow cytometry approach using
in vivo reporters for lamellocytes as well as for plasmatocytes, the main hemocyte
type in healthy larvae. We found that two different blood cell lineages, the plasmatocyte
and lamellocyte lineages, contribute to the generation of lamellocytes in a demand-adapted
hematopoietic process. Plasmatocytes transdifferentiate into lamellocyte-like cells
in situ directly on the wasp egg. In parallel, a novel population of infection-induced
cells, which we named lamelloblasts, appears in the circulation. Lamelloblasts proliferate
vigorously and develop into the major class of circulating lamellocytes. Our data
indicate that lamellocyte differentiation upon wasp parasitism is a plastic and dynamic
process. Flow cytometry with in vivo hemocyte reporters can be used to study this
phenomenon in detail.
