PD-1 or PDCD1 (Programmed cell death 1), also known as CD279, is a cell surface receptor expressed on T cells and pro-B cells, which was firstly isolated from a murine T cell hybridoma and a hematopoietic progenitor cell line in 1992. It is an inducible immune modulatory receptor and belongs to a member of the CD28 receptor family. The expression of PD-1 is significantly involved in programmed cell death of lymphocytes. PD-1 has two ligands that are PD-L1 (B7-H1 or CD274) and PD-L2 (B7-DC or CD273). Upon their interactions, PD-1 plays an active role in negative regulation of T cell responses to antigen stimulation and maintaining peripheral tolerance. Thus, it is regarded as an important immune checkpoint for therapeutic antibodies against cancers.
HER2 (Human epidermal growth factor receptor 2), also known as ERBB2, CD340 (cluster of differentiation 340), proto-oncogene Neu, is a membrane tyrosine kinase and has strongest catalytic kinase activity in HER family. Among four HER family proteins, HER2 functions as the most active signaling complex after dimerization with other members. It is encoded by the oncogene ERBB2, which is located at the human chromosome 17q12. HER2 participates in normal cell growth and found on some types of cancer cells. Its amplification is the major pathway causing HER2 receptor overexpression. HER2 is amplified in approximately 15%-20% of breast cancers and functions as the primary driver in breast tumor development and progression. The overexpressed HER2 is regarded as a valuable target in clinic therapeutics.