**9. Importance of in-vitro model infection in Drosophila**

The Drosophila S2 cells were first discovered by I. Schneider in 1972 [104]. S2 cells are derived from primary cell culture of late phase embryo of *Drosophila melanogaster*. S2 cells are macrophage like cells potentially grows in serum free medium as non-adherent suspension. S2 cells can express variety of heterologous proteins, upto 12 proteins could be co-expressed at a time in highly controlled manner, doubling at a rate equivalent to any cell lines derived from human cancerous cell line [105]. These cells do not form coherent clusters with no noisy gene expression profiles by maintaining uniformity during expression and chromosomal aneuploidy gets compensated during expression self adjusted to one gene copy number per cell unlike cancerous lineage [106]. These viable and potent cellular characteristics of S2 cell allows to be chosen for vaccine development, large scale enzyme as well as hormones production similar to Chinese hamster ovary CHO cell lines [104, 107]. The post translational glycosylation process is often not achievable in S2 cells making it disadvantageous [105]. The viral infections models are slow in inducing fatality in immuno-suppressed mutant flies, 50% death occurs after around 18–30 days post infection in live model [44, 108]. Therefore, S2 cell line model could requite certain challenges usually observed during in-vivo infection models.
