The Role of Surfactant Protein D in Neutrophil Extracellular Traps-Mediated Innate Immunity

Neutrophils extracellular traps (NETs) efficiently trap and kill invading microbes, but also inflict damage to the host. However, the mechanisms and the host factors that regulate NETosis are not clearly understood. Surfactant protein D (SP-D) is a soluble innate immune molecule present on mucosal surfaces, and functions as a pattern-recognition molecule. For my doctoral research project, I hypothesized that SP-D augments neutrophil-mediated innate immune defense by regulating NETosis and also by enhancing the anti-microbial functions of NETs. There are two previously identified pathways for NETosis: NOX-dependent and NOX-independent pathways. The results show that the NOX-dependent pathway of NETosis requires the activation of Akt and JNK, which are downstream of ROS production by NOX, to suppress the pro-apoptotic pathway. Furthermore, SP-D redirects NETosis to apoptosis by suppressing JNK activation. SP-D does not, however, affect the activation of Akt, ERK or p38 kinases. Also, this suppression of NETosis by SP-D is dose dependent, and is subject to the availability of SP-D. The presence of other soluble ligands such as bacterial LPS attenuates the ability of SP-D to suppress NETosis. SP-D, thus, may act as a barometer to control the extent of NETosis/NETs required to combat infection while minimizing tissue damage. The NOX-independent pathway has also been described previously, but its mechanism remains poorly understood. This study shows that the NOX-independent pathway, shown by its insensitivity to the inhibition by DPI, is induced by the calcium ionophore, A23187. Moreover, the NOX-independent pathyway, unlike the NOX-dependent pathway, is mediated by the activation of the calcium activated small conductance potassium (SK) channel, and it requires intact mitochondrial potential. Also, Hepoxilin A3, a derivative of arachidonic acid, can induce both the NOX-dependent and -independent pathways. Experiments using mice show that the endotracheal instillation of LPS into the mouse airways results in NET release by the recruited neutrophils in the airways, in vivo. Once NETs are formed, SP-D simultaneously interacts with NET DNA and the carbohydrate targets, and enhances bacterial trapping by NETs. Collectively, the data show that SP-D regulates NETosis and also enhances the innate immune potential of NETs.