Interestingly, the activity of caspase-3, often used as a surrogate for apoptosis, was increased in the spontaneously breathing lpr mice, and this increase reached significance in the MV + LPS group (185 ± 24 vs 480 ± 137, p < 0.05) (Figure 3-A) . PARP, a downstream target of active caspase-3, showed similar activity in the lpr and B6 mice (Figure 3-B) . Double labeling of tissue sections for TUNEL and cytokeratin revealed that in both the lpr and the B6 mice, the apoptotic cells were located primarily in the alveolar walls, but were cytokeratin-negative (Figure 3-C) . Thus, contrary to our expectations, there was increased caspase-3 activity in the lpr mice exposed to MV + LPS, and the apoptotic cells were mostly cytokeratin-negative cells localized to the alveolar walls.