Autoimmunity Caused by Cell Type-Specific Deficiency in Apoptosis.

Apoptosis in the immune system is essential for maintaining self-tolerance. Defective apoptosis causes systemic autoimmune diseases in humans and mice. However, inhibition of apoptosis in lymphocytes alone does not induce significant autoimmune responses, indicating that impaired apoptosis in other cell types plays a critical role in the breakdown of self-tolerance. To study whether apoptosis in dendritic cells (DCs) regulates self-tolerance, we generated transgenic mice expressing a potent caspase inhibitor from baculovirus, p35, in dendritic cells (DC-p35). DC-p35 transgenic mice displayed defective apoptosis in DCs, leading to DC accumulation, chronic lymphocyte activation and systemic autoimmune manifestations resembling the phenotypes in Fas-deficient lpr mice. Our results suggest that apoptosis in DCs is critical for limiting lymphocyte activation and preventing autoimmunity.

Cowpox virus CrmA, Myxoma virus SERP2 and baculovirus P35 are not functionally interchangeable caspase inhibitors in poxvirus infections

Cowpox virus (CPV) expresses the serpin (serine proteinase inhibitor) CrmA, an anti-inflammatory, anti-apoptotic protein required for production of red pocks on chicken chorioallantoic membranes (CAMs). In vitro, CrmA inhibits several caspases and granzyme B. Altering the critical P1-aspartate in the CrmA reactive centre loop to alanine resulted in a virus (CPV-CrmA-D303A) that resembled CPV deleted for CrmA (CPVΔCrmA : : lacZ); on CAMs it produced white, inflammatory pocks with activated caspase-3 and reduced virus yields, suggesting that CrmA activities are mediated via proteinase inhibition. CrmA in CPV was replaced with SERP2 from Myxoma virus (MYX) or baculovirus P35, which inhibit similar proteinases in vitro. SERP2 and P35 each blocked caspase-3-mediated apoptosis but were unable to control inflammation of CAMs. However, SERP2 and P35 restored virus yields, indicating that the decreased virus titres seen with CPVΔCrmA : : lacZ resulted from apoptosis rather than inflammation. To compare the activities of CrmA and SERP2 further, rabbits were infected with MYX recombinant viruses. Intradermal infection of rabbits with MYX was uniformly lethal, generating raised primary lesions and many secondary lesions. In contrast, deletion of SERP2 from MYX (MYXΔSERP2 : : lacZ) caused little mortality and produced flat primary lesions with few secondary lesions. Replacement of SERP2 with CrmA (MYXΔSERP2 : : CrmA) resulted in partial complementation with flat primary lesions, many secondary lesions and death in 70 % of the rabbits. Therefore, CrmA and SERP2 were not functionally interchangeable during infection of CAMs or rabbits, implying that these serpins have activities that are not evident from biochemical studies with human caspases.