As mentioned previously, GARP protein was first identified on activated Tregs and platelets [
15]. Despite the increasing knowledge about the role of GARP in Tregs, little attention has been paid until recently to the role of GARP on platelets [
24]. It is not entirely clear whether GARP plays a role in platelet activation and function since two different studies in two different animal models describe conflicting findings. The first study performed in
Danio rerio (zebrafish) demonstrated that GARP is important for thrombus initiation and hemostasis; knockdown of the
Lrrc32 gene resulted in increased spontaneous bleeding events [
95]. A second study, performed in a genetic mouse model where
Lrrc32 is specifically knocked out from platelets and megakaryocytes, shows that GARP is not necessary for thrombus formation and clot retraction, which is also confirmed by our own unpublished observations. Interestingly this last study shows that ex vivo platelet activation triggers increase in GARP surface expression, indicating that GARP might play a role in activating platelets [
35]. Using the same platelet-specific GARP knockout mouse model, we recently demonstrated that GARP enhances the activation of latent TGF-β released by platelets [
24]. Serum active TGF-β was drastically reduced in these mice, and interestingly, the similar phenotype was not observed in mice with platelet-specific deletion of
Tgfb1 gene, indicating that platelet GARP activates latent TGF-β1 secreted by cells other than by platelets [
24]. Importantly, the same study demonstrated that the platelets contribute dominantly to the activity of TGF-β in the tumor environment. Among platelet-derived soluble factors, TGF-β is one of the main mediators for the platelet-dependent tumor growth [
96,
97], which was once again confirmed by an unbiased biochemical and biophysical strategy [
24]. Accordingly, platelet-specific deletion of GARP potentiated protective immunity against both murine models of melanoma and colon cancer [
24]. This study also provides a mechanistic explanation as to why thrombocytosis is consistently associated with poor outcome in cancer [
98]. Intriguingly, blocking platelet activation pharmacologically with aspirin and clopidogrel was shown to significantly enhance the adoptive T cell therapy of murine melanoma [
24], which correlated with increased persistence and functionality of transferred donor T cells. This study lays a strong foundation for combining anti-platelet agents and immunotherapy as a novel strategy for cancer care in the future.
Beyond TGF-β, each activated platelet releases up to 80 α-granules secreting platelet-derived growth factors (PDGFs) in the tumor proximity which contribute to the platelet-cancer interaction [
96,
99]. Activated platelets, for example, secrete vascular endothelial growth factor (VEGF) that induces angiogenesis and cell migration [
100]. Additionally, multiple inflammatory cytokines are released by activated platelets such as IL-1, IL-6, granulocyte macrophage colony stimulating factor (GM-CSF) [
101], and CD40L [
102]. It is not known whether GARP plays a role in the release and/or activation of any other growth factor or cytokines produced by active platelets other than TGF-β, this intriguing possibility deserves further investigation.