TNF and infection: We propose that the limited engagement of the PD-L1/PD-1 axis during infection with T209L DENV is mainly driven by a blunted pro-inflammatory response early post-infection whereby lower levels of key cytokines including IL-6, IFN-γ and TNF-α limit PD-L1 upregulation on a variety of innate immune and non-immune cells (Bardhan et al, 2016; de Kleijn et al, 2013; Francisco et al, 2010; Jiang et al, 2019; Wilke et al, 2011).