Figure 3.

Bystander activation of alloreactive T cells by “virus-licensed” APCs. Upon viral infection, detection of pathogen-associated molecular patterns by PRRs can stimulate the production of inflammatory cytokines such as IFN-α/β, TNF-α, and IL-6. These cytokines activate alloantigen-processing APCs in a paracrine or autocrine fashion to upregulate MHC classes I and II, as well as costimulatory molecules, such as CD80 and CD86. The heightened expression of costimulatory molecules elicits the proliferation and differentiation of alloreactive T cells.