Antigen presentation by human fetal astrocytes with the cooperative effect of microglia or the microglial-derived cytokine IL-1

Abstract

Antigen presentation by endogenous glial cells is postulated to regulate reactivity of immune cells that gain entry into the CNS. We have previously observed, using a mixed lymphocyte reaction (MLR) system, that adult human- derived microglia can function as antigen-presenting cells (APC) for immediately ex vivo CD4+ T cells in a primary MLR (1° MLR) whereas astrocytes could not. We have now found that fetal human astrocytes can support CD4+ T cell proliferation in the presence of exogenous human recombinant (r) IL-2, and that astrocytes can support the continued proliferation of CD4+ T cells previously sensitized to sister astrocyte cultures in a secondary MLR (2° MLR). Additionally, adult human microglia, seeded into the nonpriming astrocyte: CD4+ T cell cocultures at non-T cell- stimulatory concentrations of 1000-5000 microglial cells per well, could reverse the inability of astrocytes to present antigen in the 1° MLR. To examine the cellular basis for the inability of human astrocytes to function as APCs in the 1° MLR, astrocyte- and microglial-enriched populations were established from human embryonic and adult brain, respectively, and analyzed for their ability to synthesize cytokines potentially relevant as accessory signals in the MLR. Microglia had transcript as determined by the reverse transcriptase-polymerase chain reaction (RT-PCR) and protein as determined by bioassay for IL-1α, IL-6, and TNFα. Human fetal astrocytes had transcript for IL-6 but not for IL-1α or TNFα under basal culture conditions and following IFNγ stimulation. The addition of human rIL-1 from 1-50 U/ml could reverse the inability of astrocytes to present antigen in the primary MLR. These studies demonstrate that although in vitro highly enriched cultures of astrocytes absent of microglia cannot present antigen to immediately ex vivo blood-derived CD4+ T cells in the MLR, in situ, with the cooperative help of microglia-derived cytokines or accessory surface molecules, astrocytes may function as central nervous system APCs.