Differential abilities of central nervous system resident endothelial cells and astrocytes to serve as inducible antigen-presenting cells.

Microglial cells and astrocytes are capable of processing and presenting antigens for efficient activation of T cells. However, the antigen-presenting function and role of cerebrovascular endothelial cells (CVEs) in central nervous system inflammatory responses remain controversial. We compared the expression of necessary accessory molecules and the functional antigen-presenting capacity of cloned SJL/J CVEs and primary astrocytes in response to the pro-inflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). Astrocytes and CVEs up-regulated major histocompatibility complex (MHC) class II, and primarily B7-1 as opposed to B7-2, in response to IFN-gamma. TNF-alpha inhibited the IFN-gamma-induced up-regulation of MHC class II on CVEs correlating to a decrease in the mRNA for the class II transactivator (CIITA), whereas CIITA expression in astrocytes was unaffected. Unlike astrocytes, CVEs did not elicit significant MHC class II-restricted T-cell responses. Furthermore, we have found that CVE monolayers are altered following T-cell contact, implicating CVE/T-cell contact in the breakdown of the blood-brain barrier during neuro-inflammatory responses.

CD40/CD40L interaction is essential for the induction of EAE in the absence of CD28-mediated co-stimulation.

CD28 provides a co-stimulatory signal critical for optimal T cell activation. We and others have shown that the B7/CD28 co-stimulatory pathway is a major regulatory pathway for the control of immune responses. Experimentally induced models of autoimmunity have been shown to be prevented or reduced in intensity in mice deficient for CD28. Here, we show that EAE and accompanying neuroantigen-specific immune responses are drastically reduced in the absence of CD28. However, we go on to show that EAE can be induced in CD28-deficient mice following two immunizations. After re-immunization, CD28-deficient mice develop severe EAE with myelin-specific responses equal to those of wildtype controls, and extensive demyelination in the spinal cord. Treatment of CD28-deficient mice with anti-CD40L at the time of immunization significantly reduced DTH responses and prevented the development of EAE following two immunizations, indicating a critical role for CD40/CD40L signaling in the absence of CD28. Taken together, our results indicate that CD28-mediated co-stimulation does not regulate immunological anergy. Instead, CD28 appears to adjust the threshold for activation and expansion of autoreactive cells.