Delayed hypersensitivity (DH), the prototypical form of cell-mediated immune responsiveness, is mediated with the participation of considerable nonspecific inflammation which necessarily disrupts the anatomic integrity of involved and adjacent tissues. Damage of this type is of minor consequence to many visceral and cutaneous organs, but is of devastating consequence for organs such as the eye and the brain. At least in the case of the eye, the organ is remarkably adept at regulating the immune system's ability to respond to intraocular antigens by selectively down-regulating both the induction and expression of delayed hypersensitivity while leaving other effector modalities intact. This ability of the eye to selectivity down-regulate systemic DH responses to intracamerally inoculated antigens is known as anterior chamber-associated immune deviation (ACAID) and is mediated in part by antigen-specific regulatory T cells. Recent work suggests that macrophages (M phi) that reside in the iris and ciliary body can migrate out of an antigen-bearing eye and activate regulatory T cells within the spleen. In an effort to understand the mechanism by which intraocular M phi interact with antigen in the anterior chamber of the eye (AC) and subsequently induce splenic regulatory cells in ACAID, we have investigated what role, if any, the AC microenvironment itself plays in ACAID induction. The results reveal that CD45- parenchymal iris/ciliary cells secrete a soluble factor(s) locally and into the aqueous humor which endows resident, mature M phi with ACAID-inducing capabilities. Mice receiving infusions of these altered, antigen-pulsed M phi are incapable of mounting a significant DH response following immunization with antigen in adjuvant. Importantly, the ACAID-inducing effect is achieved when conventional, extraocular M phi are exposed in vitro to a soluble factor present in aqueous humor or culture SN from iris and ciliary body cells. Further investigations into the identity of this factor reveal it to be transforming growth factor-beta (TGF-beta). The role of TGF-beta in the generation of ACAID, as well as the implications of these findings to an understanding of immunologic privilege in general, are discussed.

Potential applications of neural stem cells (NSCs) for transplantation requires understanding myosin heavy chain (MHC) expression and the ability of T cells and natural killer (NK) cells to recognize this progenitor population. Cells from the cortices of day‐13 embryonic (E13) B6 (H‐2b) mice were explanted and cultured to expand NSCs. Analysis of P2‐P17–cultured cells using anti‐MHC class I/II monoclonal antibodies (mAbs) showed marginal expression of both products. Although recombinant murine interferon‐gamma (rmIFNγ) exposure did not alter the multipotential capacity of these stem cells, titration of mrIFNγNSC cultures demonstrated that MHC molecules could be strongly upregulated after addition of 3 ng/ml rmIFNγ for 60 hours. To assess the susceptibility of NSCs with low or absent versus high levels of MHC expression to lysis by cytotoxic T lymphocyte (CTL) and NK populations, untreated and rmIFNγ‐treated NSC target cells were examined. Untreated NSCs were not recognized by BALB/c (H‐2d) allospecific anti‐H‐2b CTL, consistent with the mAb findings; however, upregulation of MHC products on both early and later passaged NSCs resulted in their efficient lysis by CTL. NK cells were prepared from syngeneic B6 or allogeneic BALB/c mice. Although NK cells effectively killed control YAC‐1 target cells, these effectors did not kill MHC‐deficient (or expressing) NSC targets. Thus, similar to hematopoietic, embryonic, and mesenchymal stem cell populations, unmanipulated NSCs are not readily killed by T and NK cells. These findings suggest that following transplant into syngeneic or allogeneic recipients, NSCs may exhibit diminished susceptibility to clearance by host T‐ and NK‐cell populations.

IL-2 and IL-15 are thought to be important cytokines for T cell-dependent immune responses. Mice deficient in IL-2, IL-2Rα, and IL-2Rβ are each characterized by a rapid lethal autoimmune lymphoproliferative disorder that complicates their use in studies aimed at investigating the role of these cytokines and receptors for immune responses in vivo. We have previously characterized a novel transgenic (Tg) mouse on the IL-2Rβ−/− genetic background (Tg−/− mice) that lacks autoimmune disease but still contains peripheral T cells that are nonresponsive to IL-2 and IL-15. In the present study, these mice were used to investigate the extent by which IL-2 and IL-15 are essential for T cell immunity in vivo. Tg−/− mice generated near normal primary and secondary Ab responses to OVA, readily mounted first and second set allogeneic skin graft rejection responses, and developed primary and recall CD8 T cell responses to vaccinia virus. However, Tg−/− mice generated a slightly lower level of IgG2a Abs to OVA, exhibited a somewhat delayed first set skin graft rejection response with lower allo-specific CTL, and developed a significantly lower number of IFN-γ-producing vaccinia-specific CD8+ T cells. Thus, although T effector function is somewhat impaired, T cell immunity is largely functional in the absence of IL-2- and IL-15-induced signaling through IL-2Rβ.

Qa-2, a murine class Ib major histocompatibility complex (MHC) molecule, is a possible functional homolog of human leukocyte antigen G (HLA-G). Both molecules have been implicated in immunoregulation and embryonic development and both occur in membrane-bound and soluble isoforms that arise by alternative splicing. Soluble splice variants have been implicated in the reproductive functions of HLA-G. While soluble variants of Qa-2 have been previously detected in T lymphocytes, we now demonstrate the presence of mRNA for one of the two known soluble forms of Qa-2 in eight-cell embryos and in blastocysts. Qa-2 is glycosylphosphatidy-linositol (GPI) linked in the outer leaflet of the cell membrane and is found in lipid raft microdomains where other raftassociated proteins transduce signals into the cell. In contrast, HLA-G has a truncated six amino acid cytoplasmic tail. By fluorescence co-localization in JEG-3 cells, using fluorescent cholera toxin β subunit (a lipid raft marker) and anti-HLA-G antibody, we have demonstrated that membrane-bound HLA-G also localizes to lipid rafts, consistent with functional homology between the two molecules. Finally, our experiments in which we have purified Qa-2 and transferred it via a process known as protein painting to Qa-2 negative cells represent a model for potential therapy involving HLA-G.

Reduced-intensity conditioning regimens for transplant recipients have heightened awareness of immunologic resistance to allogeneic bone marrow transplants (BMT). Although T cell-mediated cytotoxicity has been assumed to play a role in the resistance against donor allogeneic hematopoietic stem and progenitor cell grafts, several studies have reported relatively unimpaired resistance by recipients who lack perforin, Fas ligand (FasL), and other cytotoxic mediators. This study compared the early kinetics of T cell-mediated resistance in B6 (H2b) cytotoxically normal versus deficient recipients after transplantation with major histocompatibility complex-matched, minor histocompatibility antigen (MiHA)-mismatched allogeneic marrow grafts. Wild-type B6 or cytotoxic double-deficient perforin-/-/gld+/+ (B6-cdd) mice were sensitized against major histocompatibility complex-matched BALB.B or C3H.SW (H2b) MiHA and transplanted with a high dose (1 x 10(7)) of T cell-depleted bone marrow. CD8 T memory cells were shown to be present in recipients before BMT, and anti-CD8 monoclonal antibody infusion abolished resistance, thus demonstrating that CD8 T cells are the host effector population. Donor-committed and high proliferative potential progenitor numbers were markedly diminished by 48 hours after transplantation in both wild-type B6 and B6-cdd anti-donor MiHA-sensitized recipients. These observations indicate that the resistance pathway used in the cytotoxic deficient mice was both potent and rapidly induced--consistent with a CD8 memory T-cell response. To examine the role of Tumor necrosis factor-like weak inducer of apoptosis (TWEAK)- and TL1A-mediated cytotoxicity in this strong resistance, newly generated monoclonal antibodies specific for these ligands were administered to B6-cdd recipients sensitized to donor antigens. Recipients of syngeneic B6-gfp bone marrow exhibited significant donor colony-forming unit numbers after BMT. In contrast, low or absent colony-forming unit levels were detected in allogeneic recipients, including those that lacked perforin and FasL and that received anti-TWEAK, anti-tumor necrosis factor-related apoptosis-inducing ligand, and anti-TL1A monoclonal antibodies. These findings extend previous observations by demonstrating the existence of a rapidly effected resistance pathway mediated by memory CD8 effector T cells independent of the 2 major pathways of cytotoxicity. Together with previous findings, these results support the notion that effector cells derived from memory CD8 T-cell populations can mediate strong resistance against donor allogeneic MiHA-disparate hematopoietic engraftment by using a mechanism that is independent of the contribution of perforin, FasL, and the known death ligand receptor pathways.