Release of soluble Fc gamma RII/CD32 molecules by human Langerhans cells: a subtle balance between shedding and secretion?

Freshly isolated human Langerhans cells (LC) express two forms of Fc gamma RII: a membrane-associated form detected by monoclonal antibody (MoAb) anti-CD32, which recognize an extracytoplasmic epitope of the molecule, and a soluble secreted form, whose existence is suggested by reverse transcriptase-polymerase chain reaction (RT-PCR) experiments. Indeed, RT-PCR performed on total LC RNA reveals the presence of two Fc gamma RIIA mRNA, one encoding the FC gamma RIIA with a transmembrane region (membranous form) and the other without this region (soluble form). Densitometry studies performed on the two PCR products reveal that the ratio between the membranous form and the soluble secreted form is about 1.5. LC maintained in culture for 24-48 h lose the major part of their membrane Fc gamma RII expression (shown by flow cytometry) and release soluble Fc gamma RII molecules (revealed by dot-blot assay), but maintain the same ratio of the two Fc gamma RIIA mRNA. The disappearance of the membrane-associated Fc gamma RII may be explained either by modification of its recycling pathway or by proteolytic cleavage of the receptor at the cell surface. Thus, soluble Fc gamma RII molecules generated during LC culture may result from proteolytic cleavage of the cell-surface receptor and/or secretion of a soluble form derived from the translation of an alternate spliced mRNA. Interestingly, addition of TNF-alpha (10 ng/ml) to the culture medium i) maintains the expression of the membranous form, which can be detected on the LC surface at the same level as on freshly isolated LC, and ii) reverses the ratio (to 0.6) of the two Fc gamma RII mRNA, the mRNA encoding the soluble form becoming predominant. Thus, TNF-alpha seems to modify the expression of the Fc gamma RII at the mRNA level, favoring the secretion of soluble Fc gamma RII molecules, and changes the fate of the membranous Fc gamma RII.

CD1 expression is not affected by human peptide transporter deficiency.

Conventional major histocompatibility complex class I molecules are highly polymorphic and present peptides to cytotoxic T cells. These peptides derive from the proteolytic degradation of endogenous proteins in the cytosol and are translocated into the endoplasmic reticulum by a peptide transporter consisting of two transporter associated with antigen processing (TAP) molecules. Absence of this transporter leads to the synthesis of unstable peptide free class I molecules that are weakly expressed on the cell surface. Mouse nonconventional class I molecules (class Ib) may also present TAP-dependent peptides. In humans, CD1 antigens are nonconventional class I molecules. Recently, we characterized a human HLA class I deficiency resulting from a homozygous TAP deficiency. We show here that CD1a and -c are normally expressed on epidermal Langerhans cells of the TAP-deficient patients, as are CD1a, -b, and -c on dendritic cells differentiated in vitro from monocytes. Moreover, the CD1a antigens present on the surface of the dendritic cells are functional, since they internalize by receptor-mediated endocytosis gold-labeled F(ab')2 fragments of an anti-CD1a mAb. This suggests either that CD1 molecules are empty molecules, that they are more stable than empty conventional class I proteins, or that CD1 molecules present TAP-independent peptides.

[Langerhans cells and presentation of antigens]. / Cellules de Langerhans et présentation d'antigènes.

Epidermal Langerhans cells express only very few Class I major histocompatibility complex (MHC) antigens, whose ability to present peptides released from the breakdown of endogenous proteins has not been investigated to date. Langerhans cells strongly express a "nonconventional" Class I molecule, the CD1a antigen. The role played by this antigen on the surface of Langerhans cells remains unelucidated: either release or uptake of peptides derived from the body has been speculated. Langerhans cells also express Class II MHC antigens and in vitro freshly recovered Langerhans cells are capable of capturing antigens, processing them and presenting the resulting peptides associated with Class II MHC molecules to immunocompetent cells. This property is not, however, permanent. Cultured Langerhans cells are no longer capable of processing antigens because they lose their ability to (i) acidify endosomes and (ii) produce the alpha, beta and invariant chains of class II MHC molecules. Cultured Langerhans cells acquire the capacity of stimulating T lymphocytes. This contrast between the in vitro properties of freshly recovered and cultured Langerhans cells may reflect in vivo differences between epidermal Langerhans cells and Langerhans cells which have migrated to regional lymph nodes.

Membrane and soluble Fc gamma RII/III modulate the antigen-presenting capacity of murine dendritic epidermal Langerhans cells for IgG-complexed antigens.

Murine dendritic epidermal Langerhans cells (LC) are APC. This implies that LC take up, process, and present Ag to T cells. One way of doing so that could allow Ag internalization is provided by the low affinity receptors for the Fc region of IgG (Fc gamma R), which murine LC are known to express, although their isoform(s) and function(s) have not been defined. By using molecular biology and biochemical approaches, we demonstrated that LC expressed Fc gamma RIIb2 and Fc gamma RIII. Furthermore, LC internalized Fc gamma R by receptor-mediated endocytosis, as observed with gold-labeled anti-Fc gamma RII/III mAb or immune complexes. We demonstrated the biologic relevance of this process by observing that Fc gamma R-mediated Ag internalization improved by approximately 300-fold the Ag-presenting capacity of LC to T cells. Moreover, analysis of cell culture supernatants showed that two forms of soluble Fc gamma R (sFc gamma R) were released by LC: the first most probably was the secreted transmembrane-deleted Fc gamma RII isoform, Fc gamma RIIb3, and the second was a soluble receptor probably derived from the membrane-associated Fc gamma RII/III. The ability of two recombinant forms, corresponding to the two sFc gamma R released by LC, to inhibit Fc gamma R-mediated presentation enhancement was assayed. Preincubation of IgG-complexed Ag with either rsFc gamma R led to a dose-dependent decrease in the Ag-presenting capacity of LC. Taken together, our results suggest that, in vivo, LC express membrane Fc gamma R, which increase their Ag-presenting capacity for IgG-complexed Ag, and release sFc gamma R, which might be able to modulate this Ag presentation.

Human epidermal Langerhans cells secrete a soluble receptor for IgG (Fc gamma RII/CD32) that inhibits the binding of immune complexes to Fc gamma R+ cells.

Langerhans cells (LC) express Fc gamma RII on their cell surface. In this paper, we demonstrate that these cells also release soluble Fc gamma RII (sFc gamma RII) molecules. LC express transcripts encoding a membrane-associated receptor and a transmembrane-deleted Fc gamma RIIA. The latter form was identified in LC culture supernatants using specific antibodies. CHO cells, transfected with LC-derived cDNA encoding the transmembrane-deleted Fc gamma RIIA, secrete sFc gamma RIIA that include the intracellular domain and exhibit the same backbone as the protein identified in LC supernatants. Secreted sFc gamma RIIA exhibits the same pattern of binding to human and mouse IgG subclasses as do membrane Fc gamma RII and inhibits the binding of immune complexes to Fc gamma RII+ cells. In addition, CHO cells expressing the membrane-associated Fc gamma RIIA release truncated and unstable Fc gamma RIIA molecules that lack the intracellular domain. Thus, sFc gamma RII can result from shedding of membrane molecules and/or from secretion of soluble receptors lacking the transmembrane domain.