Programmed death-1 ligand 2 (PD-L2) expression extends beyond macrophages/dendritic cells to B-1 B cells, a distinct B-cell lineage that is responsible for natural immunoglobulin and which is repertoire skewed toward autoreactive specificities. PD-L2 expression is constitutive in B-1 cells, whereas it is inducible in other cell types, suggesting that PD-L2 is regulated differently in the former versus the latter, and this proved to be the case, both in transcription and promotion. B-1 cells express a PD-L2 transcript that lacks exon 1, in contrast to macrophages/dendritic cells for which exon1 is included, reflecting a unique start site upstream of exon 2. PD-L2 transcription in B-1 cells is regulated by a novel intronic promoter located between exons 1 and 2. This intronic promoter binds Octamer binding protein 1 (Oct1) and Oct2, and although these transcription factors are present in all B cells, Oct2 binding is found in vivo only in B-1 cells and not PD-L2-negative B-2 cells. Moreover, the proximal promoter upstream of exon 1 that is active in macrophages is inactive in B-1 cells. Thus, PD-L2 expression is regulated by two different promoters that function in a lineage-specific manner, with the B-1-specific promoter being constitutively active as a result of Oct1 and Oct2 binding.
Animals B-Lymphocytes