The expression of MHC class II (Ia) antigens on mouse keratinocytes following epicutaneous application of contact sensitizers and irritants

Keratinocyte-intrinsic MHCII expression controls microbiota-induced Th1 cell responses

The cross-talk between the microbiota and the immune system plays a fundamental role in the control of host physiology. However, the tissue-specific factors controlling this dialogue remain poorly understood. Here we demonstrate that T cell responses to commensal colonization are associated with the development of organized cellular clusters within the skin epithelium. These organized lymphocyte clusters are surrounded by keratinocytes expressing a discrete program associated with antigen presentation and antimicrobial defense. Notably, IL-22-mediated keratinocyte-intrinsic MHC class II expression was required for the selective accumulation of commensal-induced IFN-γ, but not IL-17A-producing CD4⁺ T cells within the skin. Taking these data together, this work uncovers an unexpected role for MHC class II expression by keratinocytes in the control of homeostatic type 1 responses to the microbiota. Our findings have important implications for the understanding of the tissue-specific rules governing the dialogue between a host and its microbiota.

Self-Antigen Presentation by Keratinocytes in the Inflamed Adult Skin Modulates T-Cell Auto-Reactivity

Keratinocytes have a pivotal role in the regulation of immune responses, but the impact of antigen presentation by these cells is still poorly understood, particularly in a situation where the antigen will be presented only in adult life. Here, we generated a transgenic mouse model in which keratinocytes exclusively present a myelin basic protein (MBP) peptide covalently linked to the major histocompatibility complex class II β-chain, solely under inflammatory conditions. In these mice, inflammation caused by epicutaneous contact sensitizer treatment resulted in keratinocyte-mediated expansion of MBP-specific CD4(+) T cells in the skin. Moreover, repeated contact sensitizer application preceding a systemic MBP immunization reduced the reactivity of the respective CD4(+) T cells and lowered the symptoms of the resulting experimental autoimmune encephalomyelitis. This downregulation was CD4(+) T-cell-mediated and dependent on the presence of the immune modulator Dickkopf-3. Thus, presentation of a neo self-antigen by keratinocytes in the inflamed, adult skin can modulate CD4(+) T-cell auto-aggression at a distal organ.

Dendritic epidermal gamma/delta T cells (DETC) activated in vivo proliferate in vitro in response to Mycobacterium leprae antigens

BACKGROUND

gamma/delta T-cell receptor (TCR)+ dendritic epidermal T cells (DETC) are part of a primitive defense system in the skin; they are capable of responding only to a limited number of antigens. The aim of the present study was to test whether DETC can proliferate in vitro in response to antigens of Mycobacterium leprae.

METHODS

DETC were obtained from CBA mouse ear skin by trypsinization and Histopaque gradient centrifugation. The resulting epidermal cell suspension contained up to 20% DETC, as analyzed by the fluorescence activated cell sorter (FACS) after staining with anti-Thy-1 or anti-gamma/delta TCR monoclonal antibodies (mAbs). The freshly isolated cells, or DETC cultured up to 4 weeks with interleukin-2 (IL-2), were exposed in vitro for up to 6 days to varying doses of the following M. leprae antigens: (1) integral (live) M. leprae bacilli; (2) Dharmendra antigen; and (3) PGL-1 (phenolic glycolipid of M. leprae). The DETC response was assessed by tritiated thymidine (3H-TdR) incorporation.

RESULTS

The freshly isolated DETC, or DETC cultured up to 4 weeks with IL-2, did not respond significantly to any of the M. leprae antigens, although at the same time they were able to respond vigorously to concanavalin A (Con A), as positive control. If, however, DETC were isolated from skin, painted 7 days before with croton oil (10 microL/cm2 to cause irritant dermatitis, they were able to respond to all M. leprae antigens by a 3-4-fold incrase in the 3H-TdR uptake. The most effective stimulator was a 1 : 1 mixture of Dharmendra and PGL-1 (0. 01 microg/mL), which was as effective as 10-fold higher doses of either antigen alone. Cell counts confirmed that increased DNA synthesis was associated with cell proliferation. Experiments employing alpha/beta-TCR CBA murine spleen cells and epidermal cell suspension treated with anti-gamma/delta or antialpha/beta mAbs + C' proved that only the gamma/delta DETC were the responder cells to M. leprae antigens.

CONCLUSIONS

The results suggest that activation of DETC in vivo may make them responsive to M. leprae antigens. A significant increase in the number of class II major histocompatibility complex (MHC) positive, nondendritic cells was observed in the croton oil-treated epidermis. We hypothesize that croson oil-induced upregulation of class II MHC expression, which endows epidermal cells with antigen-presenting capabilities, might be an important factor in vivo in delivering an immunogenic signal to resident DETC in the skin.

Antigen-presenting cell function of epidermal cells activated by hapten application

Exposure to haptens initiates a series of immune and inflammatory reactions that cause migration of epidermal Langerhans cells (LC) to draining lymph nodes, antigen processing, and presentation to T cells. In the present study, the antigen-presenting cell (APC) function of epidermal cells (EC) following hapten application was determined using a cell transfer system. This function of EC in inducing contact sensitivity (CS) in the recipient mice appeared as early as 6 h after hapten painting, and reached its maximum at 24 h. The amount of hapten on EC did not correlate with the function, i.e. the amount retained on the cells was greatest immediately after hapten painting and decreased over time. Several experiments were performed to identify the cell type responsible for the APC function. Through immunomagnetic bead separation, the APC function was detected in Ia- EC, as well as in unfractionated EC from hapten-painted mice. A purified population of Ia+ cells (LC) induced CS with much less efficiency than unseparated cells. Depletion of LC by anti-Ia monoclonal antibody (mAb) and complement-mediated lysis did not impair the APC function, whereas it was reduced by the depletion of Thy-1+ cells by anti-Thy-1 mAb and complement-mediated lysis. Moreover, adherent cells that were harvested from a 48-h culture of EC obtained from hapten-painted skin, and were free of contaminating LC and gamma omega T cells, had a strong capacity to induce CS. These findings indicate that keratinocytes (KC) acquire APC function as well as LC, with hapten application. Phenotypically increased expression of intercellular adhesion molecule-1 (ICAM-1) and Thy-1 on EC was observed following hapten application, whereas expression of Ia and B7/BB1 was unaltered. The APC function of EC from hapten-painted skin was dependent on ICAM-1 and Thy-1 expression, as the mAbs for these molecules reduced the capacity to induce CS. These results suggest that hapten application induces not only LC but also KC to mature functionally and become potent APCs, and that these KC exert the APC function complementarily at local sites following the migration of LC with potent APC function to the draining lymph node.

Keratinocyte expression of B7-1 in transgenic mice amplifies the primary immune response to cutaneous antigens

Resting epidermal keratinocytes do not express B7-1 and other known CD28 counterligands with costimulatory activity. The absence of these costimulators on keratinocytes correlates with their ability to preferentially induce T-cell anergy instead of T-cell activation. To test the hypothesis that keratinocytes expressing a CD28 counterligand would be more effective inducers of T-cell-mediated immune responses in skin, we prepared transgenic mice in which expression of the B7-1 costimulator was targeted to basal keratinocytes by using the human K14 promoter. Keratinocytes from the K14/B7-1 transgenic line expressed high levels of surface B7-1. No spontaneous inflammatory changes were seen in transgenic skin, but epicutaneous application of contact sensitizers to these mice elicited a stronger primary ear swelling response than in controls. Sites of initial hapten application in transgenic mice also responded much more strongly to reapplication of hapten to a remote cutaneous site. Epidermal cell suspensions from transgenic mice contained normal numbers of Langerhans cells and dendritic epidermal T cells when analyzed by flow cytometry. Systemic treatment of the transgenic mice with interferon gamma induced high levels of class II major histocompatibility complex expression on keratinocytes but was not sufficient to initiate an inflammatory response. We conclude that the constitutive expression of the B7-1 molecule in vivo on a nonprofessional antigen-presenting cell is not by itself sufficient to trigger inflammatory changes, but B7-1 expression amplifies the host immune responses after exposure to nonself antigens presented by B7-1-expressing cells.

The modulation of skin irritation

Irritant contact dermatitis (ICD) is a multifactorial disease, the onset and modulation of which depend on both endogenous and exogenous factors. Among the former, age, race, site, sex and history of dermatitis may all be important. Such variables can now readily be quantified by objective noninvasive techniques, such as measurement of transepidermal water loss (TEWL). Moreover, effects of irritants on the epidermis are related to the particular chemical properties of each molecule, contributing further to clinical heterogeneity. Release of cytokines and mediators may be initiated by a number of cells, including living keratinocytes and those of the stratum corneum, thus modulating inflammation and repair. Furthermore, differences in mechanisms of inflammation between acute and chronic ICD may exist, the former being characterized predominantly by inflammation, the latter by hyperproliferation and transient hyperkeratosis. These findings may explain the complexity and difficulty of investigating ICD. Better understanding and quantification of these mechanisms may lead to identification of high-risk individuals and more effective prevention and treatment.