FK506 protects against various immune responses and secondary degeneration following cerebral ischemia.

The immunosuppressant FK506 (1 mg/kg, i.p.) reduces the infarct size following 90 min occlusion of the middle cerebral artery (MCAo) in adult rat brain. Here we have investigated the effect of FK506 on cerebral immune cells that are considered to contribute to neurodegeneration. FK506 substantially attenuated the response of resident and peripheral immune cells following transient ischemia. Between 24 hr and 5 days after MCAo, FK506 reduced the T-cell infiltration in the infarct area as well as the presence of activated and/or phagocytic OX-18, OX-42, GSA-IB4, Iba1, and ED1 positive microglia/macrophages. FK506 also lowered the protein levels of TNFalpha and IL-2 in ischemic brain areas. Repetitive application of FK506 over 20 days attenuated the activation of microglia in the substantia nigra (SN), an area of secondary degeneration. Importantly, FK506 conferred also lasting protection of the neurons of SN; these neurons degenerate by withdrawal of neurotrophic factors from the striatum that undergoes necrotic death as part of the ischemic core. To understand the molecular basis of FK506 effects in cerebral immune cells, we determined in primary postnatal day 0/1 (P0/P1) microglia (i) the expression of the FK506 binding proteins FKBP12, FKBP52, and FKPB65 and (ii) that FK506 (1-100 ng/mL) lowered the number of resting or lipopolysaccharide stimulated microglia as well as we induced the lipopolysaccharide release of TNFalpha in a dose-dependent manner. In summary, FK506 confers rescue of brain tissue following cerebral ischemia not only by neuronal protection, but also by suppression of microglial activation and peripheral immune responses.