Microglia induce myelin basic protein-specific T cell anergy or T cell activation, according to their state of activation

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Abstract
Microglial cells are non‐professional antigen‐presenting cells (APC) the function of which is still controversial. Here, we studied the function of microglia derived from H‐2u mice. We show that these microglia express a low level of B7.2 and CD40 and, interestingly, lack surface expression of B7.1. Resting and IFN‐γ‐activated microglia were unable to activate naive and primed myelin basic protein (MBP)‐specific CD4+ T cells in the presence of MBP and encephalomyelitic MBP Ac1‐11 peptide. Furthermore, in the presence of Ac1‐11 peptide, CD4+ TCR‐transgenic T cells became anergized. Microglia became professional APC only after a multistep activation process involving both stimulation through cytokines [granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) and IFN‐γ] and cognate signaling (B7‐CD28 and CD40‐CD40 ligand interactions). As such they were able to present MBP to both unprimed and primed T cells. Co‐culture of microglia with GM‐CSF up‐regulated co‐stimulatory molecules, in particular B7.1. Additional activation with IFN‐γ induced MHC class II and CD40 up‐regulation. CD40‐CD40 ligand interaction significantly enhanced microglial ability to prime TCR‐transgenic T cells and was essential for presentation of MBP to in vivo primed non‐transgenic T cells. We propose that microglia may serve different functions under different inflammatory conditions, depending on the cytokine milieu and the type of cognate interaction they are involved in.