The programmed cell death-1 receptor (PD-1) is an immune checkpoint inhibitor which is expressed on the surface of immune effector cells. It is activated mainly by PD-L1 which can be expressed by all human cells. The PD-1/PD-L1 pathway plays a subtle role in maintaining peripheral T-lymphocyte tolerance and regulating inflammation. In cancer, the expression of PD-L1 seems to be one of the major immune escape mechanisms. Many studies have shown efficacy of blocking PD-1 or PD-L1 with specific antibodies like pembrolizumab or atezulizumab. In breast cancer, potential response was demonstrated in metastatic triple-negative breast cancers.

The antibacterial peptide SA-FF22, produced by the pathogen Streptococcus pyogenes strain FF22 was purified and features of its primary and secondary structure were characterised. Mass spectrometry demonstrated the pure peptide had a mass of 2794Da while, amino acid analysis revealed the presence of the unusual, thioether amino acids lanthionine and 3-methyllanthionine ; thus SA-FF22 is a member of the group of antibacterial polypeptides termed lantibiotics. Furthermore, amino acid sequencing showed a unique sequence which was blocked at position 23 by a residue of the unsaturated amino acid 2,3-didehydrobutyrine. Carboxypeptidase-Y digestion could be used to demonstrate that serine occupies the C-terminal position only after complete oxidation of the thioether amino acid bridges, suggesting that the three-dimensional structure of the native peptide may prevent access of the enzyme to the C-terminus. Fragmentation of the native peptide with a variety of proteolytic enzymes failed to yield a peptide containing less than all three of the cross-linked lanthionine and methyllanthionine residues and demonstrated that all three thioether bridges overlapped. Analysis of the circular dichroism of SA-FF22 in various concentrations of 2,2,2-trifluoroethanol in water, SDS micelles and in the presence of artificial phospholipid vesicles suggested that there is significant change in its secondary structure from aqueous to lipophilic environments.SA-FF22 is a bacteriocin-like inhibitory substance produced by Streptococcus pyogenes strain FF22 and which has been shown to be bactericidal for many strains of related streptococci [l]. Subsequent to its discovery, SA-FF22 was partially purified and characterised [2, 31. In these initial studies it was demonstrated that the inhibitory factor contained an essential proteinaceous component since it was inactivated by several proteolytic enzymes, was of approximately 8000Da as determined by dialysis and gel filtration, was stable in acid solution and at high temperatures but was not stable under alkaline conditions and was active against only certain Gram-positive bacteria. Further studies showed that the genetic determinant(s) that controlled SA-FF22 production appeared to be lost spontaneously in aged cultures or through the use of plasmid 'curing' agents, suggesting that SA-FF22 production was encoded on a plasmid. AdditionCorrespondence to R. W. Jack,

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