Injection of OCs into adult male flies induces a strong transcriptomic response in the host flies featuring in particular genes encoding bona fide G coupled proteins, among which the gene for methuselah like 1 is prominent. The injection is followed after a 3-d lag period, by the proliferation of the oncogenic cells. We hypothesized that through the product of mthl1 the host might control, at least in part, this proliferation as a defense reaction. Through a combination of genetic manipulations of the mthl1 gene (loss of function and overexpression of mthl1), we document that indeed this gene has an antiproliferative effect. Parallel injections of primary embryonic Drosophila cells or of various microbes do not exhibit this effect. We further show that mthl1 controls the expression of a large number of genes coding for chemoreceptors and genes implicated in regulation of development. Of great potential interest is our observation that the expression of the mouse gene coding for the adhesion G -protein -coupled receptor E1 (Adgre1, also known as F4/80), a potential mammalian homologue of mthl1, is sig-nificantly induced by B16 -F10 melanoma cell inoculation 3 d postinjection in both the bone marrow and spleen (nests of immature and mature myeloid-derived immune cells), respectively. This observation is compatible with a role of this GPCR in the early response to injected tumor cells in mice.

We have injected dish-cultured oncogenic RasV12 cells into adult male flies and analyzed by single cell transcriptomics their destiny within the host after 11 days. We identified in the preinjection samples and in the 11-day postinjection samples in all 16 clusters of cells, of which 5 disappeared during the experiment in the host. The other cell clusters expanded and expressed genes involved in the regulation of cell cycle, metabolism, and development. In addition, three clusters expressed genes related to inflammation and defense. Predominant among these were genes coding for phagocytosis and/or characteristic for plasmatocytes (the fly equivalent of macrophages). A pilot experiment indicated that the injection into flies of oncogenic cells, in which two of most strongly expressed genes had been previously silenced by RNA interference, into flies resulted in a dramatic reduction of their proliferation in the host flies as compared to controls. As we have shown earlier, the proliferation of the injected oncogenic cells in the adult flies is a hallmark of the disease and induces a wave of transcriptions in the experimental flies. We hypothesize that this results from a bitter dialogue between the injected cells and the host, while the experiments presented here should contribute to deciphering this dialogue.

Oncogenic RasV12 cells [A. Simcox et al., PLoS Genet. 4, e1000142 (2008)] injected into adult males proliferated massively after a lag period of several days, and led to the demise of the flies after 2 to 3 wk. The injection induced an early massive transcriptomic response that, unexpectedly, included more than 100 genes encoding chemoreceptors of various families. The kinetics of induction and the identities of the induced genes differed markedly from the responses generated by injections of microbes. Subsequently, hundreds of genes were up-regulated, attesting to intense catabolic activities in the flies, active tracheogenesis, and cuticulogenesis, as well as stress and inflammation-type responses. At 11 d after the injections, GFP-positive oncogenic cells isolated from the host flies exhibited a markedly different transcriptomic profile from that of the host and distinct from that at the time of their injection, including in particular up-regulated expression of genes typical for cells engaged in the classical antimicrobial response of Drosophila.