Developmental Differences between Neonatal and Adult Platelets in Regard to Immune Protein Content and Regulation of Monocyte Inflammatory Responses

Preterm neonates frequently receive platelet transfusions (PTx), owing to their high incidence of thrombocytopenia and bleeding. However, a recent large randomized trial found a significantly higher incidence of death or major bleeding among more liberally transfused preterm neonates. The mechanisms underlying these deleterious effects are unknown, but it has been suggested that they are related to the developmental differences between adult and neonatal platelets, and to a potential “developmental mismatch” occurring when adult platelets (comparatively hyperreactive) are transfused into neonates. While most studies so far have focused on the hemostatic effects of PTx, it is now clear that platelets are key modulators of immune and inflammatory responses. We hypothesized that adult platelets would be more pro-inflammatory than neonatal platelets.

To test this hypothesis, we first compared the proteome of resting cord blood (CB, n=9) and adult (n=7) platelets. Using data-independent acquisition mass spectrometry, we identified 4173 high-confidence proteins present in both adult and neonatal platelets. In a principal component analysis, neonatal and adult platelets clustered separately, and 359 proteins were present at significantly different relative abundances between the two groups (≥1.5-fold change, p<0.05). Pathway analysis of the 169 proteins significantly more abundant in the adult platelet proteome revealed enrichment for immune and inflammatory pathways, while neonatal platelets were enriched for pathways involved in metabolic processes.

Next, we compared the effects of neonatal and adult platelets on the inflammatory response of human neonatal monocytes. Platelets and monocytes were isolated from healthy adult blood or full-term CB (n=6/group). Platelet P-selectin surface expression was determined by flow cytometry, and key platelet-derived immune regulators (RANTES, β-2 Microglobulin [β2M], and TGF-β) were quantified in the supernatant of resting and activated platelets by ELISA. After washing, neonatal and adult platelets had similarly low P-selectin surface expression levels. Following thrombin activation, P-selectin surface expression was significantly higher in adult than in neonatal platelets (MFI 4010±1645 vs. 1987±816, p=0.04). Thrombin activation increased RANTES and TGF-β release from adult and neonatal platelets to similar levels. In contrast, β2M levels were significantly higher in the supernatant from activated adult compared to neonatal platelets (37.9±10.2 vs 17.9±5.0 ng/mL, p=0.0004). Next, we co-cultured activated adult or neonatal platelets with neonatal monocytes and quantified the pro-inflammatory cytokines IL-8 and MCP-1 (CCL2) in the supernatants after 18 hours. The presence of activated platelets significantly increased IL-8 production from neonatal monocytes compared to monocytes alone, 2.9±1.2-fold when neonatal platelets were added and 4.2±1.8-fold with addition of adult platelets (p=0.01 and p=0.001, respectively). MCP-1 increased to similar levels with the addition of either neonatal or adult platelets. In a regression model, both P-selectin expression and platelet-released β2M levels were significantly associated with monocyte-produced IL-8 levels (p=0.001 and p=0.04, respectively). In summary, we found that proteins related to inflammatory and immune responses are more abundant in adult compared to neonatal platelets, and that activated adult platelets express more P-selectin and release more β2M than neonatal platelets, resulting in higher IL8 production by neonatal monocytes. These findings support the hypothesis that the pro-inflammatory effects of adult platelets might contribute to the increased morbidity and mortality associated with neonatal PTx.