Instant intra-operative neutropenia despite the emergence of banded (CD16(dim)/CD62L(bright)) neutrophils in peripheral blood - An observational study during extensive trauma-surgery in pigs

Introduction: Deregulation of polymorphonuclear neutrophils (PMNs) is an essential step in the development of inflammatory complications upon trauma. Different neutrophil subtypes have been identified recently, however, the role of neutrophil subtypes in immunoregulation upon trauma is unclear. We hypothesize that extensive trauma surgery causes instant progressive heterogeneity of the blood neutrophil pool, and increased appearance of young (CD16(dim)/CD62L(bright)) neutrophils in peripheral blood.Material and methods: A standardized extensive thoraco-abdominal porcine trauma surgery model was utilized, and 12 animals were included. Blood was collected at defined timepoints and neutrophil numbers and subtypes were studied by flowcytometry. Neutrophil subtypes were identified by differences in cell surface expression levels of CD16 (Fc gamma RIII) and CD62L (L-selectin). Porcine neutrophil subtypes were further characterized after flow sorting.Results: Eleven animals survived the 3-hour surgical protocol. Neutrophil numbers dropped significantly from a mean of 8,6 +/- 3,5 x 10(6) to 2,4 +/- 1,8 x 10(6) cells/ml during 180 min, (p<0.001). Simultaneously, the blood PMN population became increasingly heterogeneous due to the appearance of new neutrophil subtypes. Cell sorting experiments and cytological analysis revealed that these porcine subtypes had specific morphological characteristics, mimicking their human counterparts. At baseline, 88% +/- 1 percent of circulatory PMNs comprised of mature (CD16(bright)/CD62L(bright)) PMNs, while at 3 h the blood PMN pool consisted of 59% +/- 2 percent of mature subtypes (p<0.001). Despite a marked drop in neutrophil levels during surgery, absolute and relative numbers of banded (CD16(dim)/CD62L(bright)) neutrophils continued to rise throughout surgery.Conclusion: Standardized extensive trauma surgery was associated with instant progressive neutropenia and increased heterogeneity of the blood neutrophil pool. Furthermore, three different neutrophil subsets in peripheral porcine blood were identified over the course of surgery. Further studies should clarify their precise role in the development of early organ failure upon extensive trauma surgery. This for the first time exemplifies experimentally the time constraints and impact of damage control surgery after severe trauma. (C) 2020 Published by Elsevier Ltd.