HLA-F Is a Predominantly Empty, Intracellular, TAP-Associated MHC Class Ib Protein with a Restricted Expression Pattern

Abstract

HLA-F is currently the most enigmatic of the human MHC-encoded class Ib genes. We have investigated the expression of HLA-F using a specific Ab raised against a synthetic peptide corresponding to amino acids 61–84 in the α1 domain of the predicted HLA-F protein. HLA-F is expressed as a β2-microglobulin-associated, 42-kDa protein that shows a restricted tissue distribution. To date, we have detected this product only in peripheral blood B cells, B cell lines, and tissues containing B cells, in particular adult tonsil and fetal liver, a major site of B cell development. Thermostability assays suggest that HLA-F is expressed as an empty heterodimer devoid of peptide. Consistent with this, studies using endoglycosidase-H and cell surface immunoprecipitations also indicate that the overwhelming majority of HLA-F contains an immature oligosaccharide component and is expressed inside the cell. We have found that IFN-γ treatment induces expression of HLA-F mRNA and HLA-F protein, but that this does not result in concomitant cell surface expression. HLA-F associates with at least two components of the conventional class I assembly pathway, calreticulin and TAP. The unusual characteristics of the predicted peptide-binding groove together with the predominantly intracellular localization raise the possibility that HLA-F may be capable of binding only a restricted set of peptides.

Biochemical Features of the MHC-Related Protein 1 Consistent with an Immunological Function

“…Conversely, we have preliminary data indicating that HFE molecules (which do not bind peptide) do not associate with the peptide-loading complex (unpublished observation). Interestingly, HLA-F molecules, of unknown peptide-binding status, were recently shown to associate with TAP (62,63). Like MR1, HLA-F molecules remain FIGURE 8.…”

Cutting Edge: Monarch-1: A Pyrin/Nucleotide-Binding Domain/Leucine-Rich Repeat Protein That Controls Classical and Nonclassical MHC Class I Genes

“…HLA-G is expressed by activated macrophages (17) and classically by trophoblasts (15), HLA-F by B cells (18), and HLA-E is ubiquitous in expression (19). A series of conserved upstream DNA sequences in the classical MHC-I promoter include the AP-1 site (20), the NF-B site, the IFN regulatory factor (IRF) binding site, and the S-X-Y module shared with MHC-II promoters (15).…”

HLA-G Transactivation by cAMP-response Element-binding Protein (CREB)

“…[1][2][3]. Among the nonclassical HLA class I molecules, HLA-E and HLA-G have been shown to be able to present peptide (3,4), whereas HLA-F is predominantly expressed as an empty intracellular molecule (5). Although there is increasing evidence that nonclassical class I molecules could be important in protection against natural killer cell-mediated responses (2,3), their exact role in T cell-mediated immune responses is still under debate.…”

“…Coinciding with the expression of HLA-G in this tissue is a general lack of classical MHC class I expression (3,5). Together, this is proposed to be of importance for materno-fetal tolerance (2,3,(11)(12)(13).…”