Cytokines derived from macrophages (M) play a critical role in the development of type 1 diabetes in the nonobese diabetic (NOD) mouse. Based on earlier findings from lupus-prone strains of inherent cytokine defects in M , NOD M were evaluated for intrinsically dysregulated cytokine production with the potential to initiate or exacerbate disease. Endotoxin-activated peritoneal M from young prediseased NOD mice produced interleukin (IL)-1 and tumor necrosis factor (TNF)-␣ levels similar to those

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... of M from a panel of control strains but reduced compared with the congenic diabetes-resistant NOR strain. IL-6 and IL-10 production were similar in NOD and NOR M, indicating that reduction in NOD IL-1 and TNF-␣ expression was selective. Nevertheless, the ratio of TNF-␣ and IL-10 production, a stringent index of normal M function, distinguished NOD from all normal strains. The most striking feature of NOD M, however, was their substantially elevated IL-12 production. This response was induced not only by endotoxin but also by bacillus Calmette-Guérin (BCG) and CD40 ligand and was associated with (and likely caused by) the enhanced and prolonged expression of p40 mRNA. Moreover, NOD M IL-12 expression appeared to be near maximally induced by lipopolysaccharide (LPS) alone, because it was only slightly enhanced by the addition of ␥-interferon, a stimulus that substantially elevated LPSinduced IL-12 production in M from normal strains. Accompanied by a unique profile of TNF-␣ and IL-10, the dramatic elevation of IL-12 expression by NOD M reflects intrinsic defects of the innate immune system with the potential to initiate and propagate the pathogenic autoreactive T-helper type 1 response characteristic of type 1 diabetes.