Polypeptides having an amino acid sequence of SEQ ID NO: 2, and having amino acid residues 9 to 27 of SEQ ID NO: 2 are disclosed, which are together named C-terminal polypeptides of CKLF1. Also disclosed is a pharmaceutical composition containing a therapeutically effective amount of the CKLF1 C-terminal polypeptide and pharmaceutically acceptable salts, carrier or excipient. Further disclosed are the polynucleotides encoding the C-terminal polypeptides of CKLF1, and vectors and host cells containing the polynucleotides; the in vitro assays for detecting the expression level of the polypeptide or polynucleotide in a test sample; and the monoclonal or polyclonal antibodies against the polypeptides or active fragments thereof. The polypeptides of the present invention can be pharmaceutically used for treating the HIV infection, allergic disease, allograft rejection, diseases in brain and autoimmune diseases.