4F2, also termed CD98, is an integral membrane protein consisting of a heavy chain linked to a light chain by disulfide bond. We have generated a monoclonal antibody to the mouse 4F2 light chain and cloned the cDNA. It encodes a mouse counterpart of rat L-type amino acid transporter-1, and induces system L amino acid transport in Xenopus oocytes in the presence of 4F2 heavy chain. Transfection studies in mammalian cells have indicated that the 4F2 heavy chain is expressed on the plasma membrane on its own, whereas the 4F2 light chain can be transported to the surface only in the presence of 4F2 heavy chain. 4F2 heavy chain is expressed diffusely on the surface of fibroblastic L cells, whereas it is localized selectively to the cell-cell adhesion sites in L cells expressing cadherins. These results indicate that the 4F2 heavy chain is associated covalently with an amino acid transporter and controls the cell surface expression as well as the membrane topology of the 4F2 heterodimer. Although 4F2 heavy and light chains are expressed coordinately in most tissues, the light chain is barely detected by the antibody in kidney and intestine, despite the presence of heavy chain in a complex form. The results predict the presence of multiple 4F2 light chains.

Human γδ T cells respond to nonpeptide Ags such as pyrophosphomonoesters and alkylamines in a γδ TCR-dependent manner in the absence of other APCs. Recently, aminobisphosphonates such as pamidronate have also been shown to activate human γδ T cells. In the present study, we indicate that activation of primary γδ T cells by pamidronate strictly depends on the presence of monocyte-lineage cells, unlike that by pyrophosphomonoesters. Thus, although pamidronate induced cell clustering, proliferation, and IFN-γ production of γδ T cells in the culture of PBMC, it failed to induce any of these activities in the culture of purified primary γδ T cells. By adding back the purified monocytes, however, both cell clustering and IFN-γ production of γδ T cells by pamidronate could be restored. The pamidronate-pulsed, but not untreated, myelomonocytic line, THP-1, was capable of activating the purified γδ T cells to produce IFN-γ, which was associated with the down-regulation of γδ TCR. Furthermore, pamidronate-pulsed THP-1 cells were significantly more susceptible to γδ T cell-mediated cytotoxicity than untreated THP-1. Also, TCR-defective Jurkat T cells transfected with γδ TCR genes produced a significant level of IL-2 in response to the pamidronate-pulsed THP-1 cells. These results have suggested strongly that human γδ T cells are functionally activated via γδ TCR by aminobisphosphonate Ag presented on the surface of monocyte lineage cells rather than directly by its free form .

Human Vγ2/Vδ2+ γδ T cells respond to low molecular-mass nonpeptide Ags in a γδ TCR-dependent manner. Although requirements of Ag presentation have remained controversial, we have indicated that specific responses of the primary γδ T cells to pamidronate were dependent on monocytic adherent cells for Ag presentation. Here, we show that human tumor cells can efficiently present aminobisphosphonate and pyrophosphomonoester compounds to γδ T cells, inducing specific proliferation and IFN-γ production. γδ TCR dependency of the response to Ag-pulsed tumor cells was confirmed by using a Jurkat line transfected with a Vγ2/Vδ2 γδ TCR. Furthermore, γδ T cells exhibited markedly enhanced cytotoxicity against the Ag-pulsed tumor cells as compared with untreated tumor cells. Survey of a number of human tumor cell lines of different origins revealed that the majority of them became susceptible for γδ T cell-mediated cytotoxicity following the Ag pulsing except for breast cancer lines so far examined, while normal PHA blast cells remained resistant. The results not only imply a unique mode of nonpeptide Ag recognition by human γδ T cells but also may provide a novel strategic clue for immunotherapy of human malignancy.

To understand the mechanisms involved in immunological tolerance to skin-associated antigens, we have developed transgenic (Tg) mice that express a model self-antigen, membrane-bound chicken ovalbumin (OVA), under the control of a keratin 14 (K14) promoter. K14-mOVA Tg mice express OVA mRNA in the epidermis, and appear normal. K14-mOVA Tg mice failed to mount T cell and delayed type hypersensitivity reactions to OVA, suggesting that the Tg mice were tolerant to OVA. Skin dendritic cells, including Langerhans cells, may contribute to the tolerance induction because migratory skin DC derived from K14-mOVA efficiently activated CD8(+) T cells from OVA-specific T-cell receptor (Va2/Vb5) Tg (OT-I) mice. OT-I cells expanded and accumulated in skin-draining lymph nodes after intravenous injected into K14-mOVA mice and exhibited activation markers. Graft-versus-host disease-like skin lesions appeared in K14-mOVA mice by day 7 after injection of OT-I cells. These studies demonstrate that K14-mOVA Tg mice are susceptible to an autoimmunelike skin disease induced by passively transferred naïve CD8(+) OVA T-cell receptor Tg T cells, and serve as a good model for understanding self-tolerance and for the investigation of the pathogenesis, treatment and potential prevention of cell-mediated autoimmune reactions in skin.