Abstract
Work by our lab and others have shown that compared to neonatal platelets, adult platelets are relatively enriched in both transcripts and proteins that are immune related, but neonatal platelets express some growth factors and enzymes at comparatively higher levels. Neonatal and adult platelet functions in thrombosis have received some research attention, but there is little understanding of the developmental roles for platelets in immune differentiation and immune cell responses to infection or injury. Furthermore, thrombocytopenic neonates receive adult platelet transfusions, but these come with significant risk of short and long-term complications, many of which are associated with immune dysfunction. Our prior work showed that transfusion of adult platelets, but not neonatal platelets, to neonatal mice induced a trafficking monocyte phenotype. We have now discovered a novel indirect mechanism of how neonatal platelets limit T cell responses that may have an impact on approaches to platelet transfusions to neonates.
Myeloid Derived Suppressor Cells (MDSCs) are a set of myeloid lineage derived cells that limit T cell activation, in part through their expression of programmed death-ligand 1 (PD-L1) that interacts with PD-1 on T cells to limit T cell responses, as well as through their production of reparative molecules (TGFb and IL-10). MDSCs are more prevalent in neonates compared to adults and MDSCs decline with age. RNA-seq on adult monocytes co-cultured for 48 hrs with releasates from neonatal or adult platelets showed that neonatal platelet co-cultured monocytes had increased expression of numerous MDSC markers, including Ptgs2, Ptges, Il10, Tnfrsf10b (DR5), and CD274 (PD-L1), amongst many others. These data were validated by qRT-PCR and flow cytometry. To determine the functional consequences of neonatal platelet-monocyte interactions, neonatal or adult platelet releasates were co-cultured with monocytes for 3 days and T cells then added to the co-culture and T cells were activated with anti-CD3/CD28. Neonatal platelet treated monocytes limited T cell activation as measured by proliferation and CD25/69 expression compared to control monocytes, while adult platelet and monocyte co-cultures had no effect on T cell activation. We next used an in vivo ovalbumin (Ova) induced asthma model, in which mice were given intravenous injections of monocytes incubated with neonatal platelet or adult platelet releasates for 3 days in vitro prior to injection. This was followed by systemic Ova priming and intra-pulmonary Ova induced T-cell activation. Mice given neonatal platelet treated monocytes had fewer activated T cells in their lungs following Ova treatment. These platelet-driven effects are dependent on neonatal platelets producing more PGE2, a described mediator of MDSC differentiation. Activated neonatal platelets produced more PGE2, expressed more of its synthesis enzyme (PTGES-1), and blocking monocyte EP4 limited neonatal platelet induced monocyte differentiation to MDSCs.
Neonatal platelets may therefore indirectly limit early post-natal T-cell responses. This has the potential to significantly impact outcomes of neonatal platelet transfusions, in which neonates are given adult platelet transfusions that may lead to adaptive immune dysfunction in these early ages of immune development.
