The disclosure found that TLT 1 directly binds to immune cells and triggers cells to express immunosuppressive phenotypes; i.e. down regulation of HLA DR and up regulation of PD L1. The induction of PD L1 and reduction of HLA DR by TLT 1 or TLT 1 derived polypeptides can therefore be useful in treating diseases associated with immune hyper reactivity such as autoimmune diseases hypersensitivity reactions transplantation rejections and graft versus host disorders.