CASP1 and influenza: Signal 1 induces the expression of the inflammasome components and IL-1β and IL-18 pro-forms, while signal 2 leads to inflammasome assembly, caspase-1 auto-cleavage and mature IL-1β and IL-18 release.14 Since the mRNA levels of IL-1β and inflammasome components were comparable in both the lungs and sorted alveolar macrophages of influenza-infected control and neutrophil-depleted mice, this would support the assertion that provision of first signal is unaltered by absence of neutrophils.