RIP-LCMV transgenic mice that express the viral glycoprotein (GP) or nucleoprotein (NP) from lymphocytic choriomeningitis virus (LCMV) under control of the rat insulin promoter (RIP) in pancreatic β-cells develop autoimmune diabetes (IDDM) after infection with LCMV. Previous reports have described that the viral infection activates naı̈ve, potentially autoreactive CD8⁺cytotoxic T-lymphocytes (CTL) that are present in the periphery of these mice, thus leading to the breaking of immunological unresponsiveness to the viral self-antigen expressed on β-cells. However, we find that adoptive transfer of such CTL that were active in vitro and in vivo into uninfected RIP-LCMV recipients rarely resulted in hyperglycemia nor in insulitis, despite their ability to home to the islets and induce peri-insulitis. These observations indicated that, in addition to activated autoreactive lymphocytes, other factor(s) were required for β-cell destruction. The present study shows that upregulation of MHC class II molecules associated with the attraction/activation of antigen presenting cells (APCs) to the islets occurs as soon as 2 days after LCMV inoculation of transgenic mice, clearly before CD4⁺and CD8⁺lymphocytes are found entering the islets (days 6 and 7 after LCMV inoculation). In contrast, although some MHC class II upregulation is also found in islets of non-transgenic mice 2–4 days after LCMV infection, no insulitis or IDDM develops and MHC is downregulated to normal (pre-infection) levels by day 7–10 in these mice. Associated with the activation of APCs and MHC upregulation observed in transgenic mice, viral (LCMV) infection of islets was detectable 2 days post-viral inoculation in some mice. Thus, β-cell destruction by activated autoreactive lymphocytes is a multifactorial process that is likely to require changes within the islet milieu or dysfunction of islets.