Permissiveness of THP-1 monocytic cells to HSV-1 infection: role of the autophagic pathway

Current research clearly shows that autophagy exerts numerous functions in vital biological processes. In particular, at distinct levels, autophagy is involved in both innate and adaptive immunity. Mechanisms triggering this process are different and affect both host permissiveness and outcome of viral infections. Herpes simplex virus type 1 (HSV-1) is known to induce autophagy in permissive cells, such as fibroblasts and neurons, by means of dsRNA-dependent protein kinase PKR that phosphorylates the eukaryotic initiation factor 2a (eIF2a). However, HSV-1 also counteracts autophagy via ICP34.5, which dephosphorylates eIF2a and inhibits Beclin 1, one of the most important players of autophagy activation. A number of studies focused on autophagy and HSV-1 in permissive cells, but less is known about semipermissive cells. Recent studies revealed a novel HSV-1-related autophagy-stimulating pathway in non permissive myeloid cells that is independent of the classical PKR/eIF2a pathway. Based on this observation, we focused our attention on the role of autophagy in the permissiveness of monocytic THP-1 cells to HSV-1 in the early phase of infection. To this purpose we utilized transfectants generated in our laboratory that over-express a murine form of Bcl-2 (THP-mBcl-2) as a central negative regulator of the autophagic process. We first characterized the capability of HSV-1 to efficiently replicate in both cellular systems. Results have shown that THP-1 cells are less permissive than THP-mBcl-2 cells, as demonstrated by the reduction of viral genes expression and in new viral particles production. Moreover, we demonstrated that HSV-1 stimulates the autophagosome formation at 1 hour post infection in THP-1 cells, independent on viral gene expression or viral DNA accumulation, as shown by LC3II and p62 dots accumulation in infected cells. Interestingly, the role of p62 in innate immunity has been emphasized by the recent finding that p62 is a downstream target of innate defense regulator-1, an antimicrobial peptide that suppresses infection. Conversely, HSV-1 inhibited the autophagosome formation later in infection, as suggested by disappearance of LC3II and p62 characteristic dots. Nucleofection of THP-1 cells with a GFP-LC3 II plasmid supported these results, showing accumulation of GFP-LC3 positive cells at 1 hour p.i and its decrease at later time points. In conclusion, our findings suggest that activation of autophagy early after infection could be one of the mechanisms that limit permissiveness of monocytic cells to HSV-1 infection compared to fully permissive cell types.