Granzyme A in Human Platelets Regulates Pro-Inflammatory Gene Synthesis by Monocytes in Aging

Dysregulated inflammation is implicated in the pathobiology of aging, yet platelet-leukocyte interactions and downstream inflammatory gene synthesis in older adults remains poorly understood. Highly-purified human platelets and monocytes were isolated from healthy younger (age<45, n=37) and older (age60, n=30) adults and incubated together under autologous and non-autologous conditions. Inflammatory gene synthesis by monocytes, basally and in the presence of platelets, was examined. Next-generation RNA-sequencing allowed for unbiased profiling of the platelet transcriptome in aging. Basal IL-8 and MCP-1 synthesis by monocytes alone did not differ between older and younger adults. However, in the presence of autologous platelets, monocytes from older adults synthesized greater IL-8 (415 vs. 92 ng/mL, p<0.0001) and MCP-1 (867150 vs. 21636 ng/mL, p<0.0001) than younger adults. Non-autologous experiments demonstrated that platelets from older adults were sufficient for upregulating inflammatory gene synthesis by monocytes. Using RNA-seq followed by validation via RT-PCR and immunoblot, we discovered that granzyme A (GrmA), a serine protease not previously identified in human platelets, is increased in aging (~9-fold vs. younger adults, p<0.05) and governs increased IL-8 and MCP-1 synthesis through TLR4 and caspase-1. Inhibiting GrmA reduced the excessive IL-8 and MCP-1 synthesis in older adults to levels similar to younger adults. In summary, human aging is associated with changes in the platelet transcriptome and proteome. GrmA is present and bioactive in human platelets, is higher in older adults, and controls inflammatory gene synthesis by monocytes. Alterations in the platelet molecular signature and downstream signaling to monocytes may contribute to dysregulated inflammatory syndromes and adverse outcomes in older adults.