Cutaneous Immune Function Research Articles

Commentary: Sport-specific Factors Impacting Solar Ultraviolet Exposure in Individuals Who Perform Outdoor Sport Activities

The epidemiologic and molecular links between ultraviolet radiation (UVR) exposure and subsequent development of both melanoma and nonmelanoma skin cancers have been well elucidated. This relationship is explained by the tendency of UVR to form DNA-damaging reactive oxygen species, increase production of local growth factors, and impair cutaneous immune function in the skin.

212 Evidence that PPARγ plays a key role in cutaneous immune function and activation of PPARγ promotes immune-mediated clearance of tumors derived from a cutaneous SCC cell line

212 Evidence that PPARγ plays a key role in cutaneous immune function and activation of PPARγ promotes immune-mediated clearance of tumors derived from a cutaneous SCC cell line

Epidermal PPARγ influences subcutaneous tumor growth and acts through TNF-α to regulate contact hypersensitivity and the acute photoresponse.

It is known that ultraviolet B (UVB) induces PPARγ ligand formation while loss of murine epidermal PPARγ (Pparg-/-epi) promotes UVB-induced apoptosis, inflammation, and carcinogenesis. PPARγ is known to suppress tumor necrosis factor-α (TNF-α) production. TNF-α is also known to promote UVB-induced inflammation, apoptosis, and immunosuppression. We show that Pparg-/-epi mice exhibit increased baseline TNF-α expression. Neutralizing Abs to TNF-α block the increased photo-inflammation and photo-toxicity that is observed in Pparg-/-epi mouse skin. Interestingly, the increase in UVB-induced apoptosis in Pparg-/-epi mice is not accompanied by a change in cyclobutane pyrimidine dimer clearance or in mutation burden. This suggests that loss of epidermal PPARγ does not result in a significant alteration in DNA repair capacity. However, loss of epidermal PPARγ results in marked immunosuppression using a contact hypersensitivity (CHS) model. This impaired CHS response was significantly alleviated using neutralizing TNF-α antibodies or loss of germline Tnf. In addition, the PPARγ agonist rosiglitazone reversed UVB-induced systemic immunosuppression (UV-IS) as well as UV-induced growth of B16F10 melanoma tumor cells in syngeneic mice. Finally, increased B16F10 tumor growth was observed when injected subcutaneously into Pparg-/-epi mice. Thus, we provide novel evidence that epidermal PPARγ is important for cutaneous immune function and the acute photoresponse.

Oral photoprotection: reducing damage from ultraviolet light and inflammation

Clinicians practising within the medical aesthetic sector commonly advise patients to apply topical sun protection factor to protect their skin from sun damage. However, it is a little known fact that certain orally consumed nutrients can play an important role in helping to protect the skin from damage induced by ultraviolet (UV) light exposure. When provided at therapeutic doses, antioxidants, probiotics, B vitamins and fish oils have all been shown to exert photoprotective effects. The mechanisms by which these nutrients can help to protect the skin against UV radiation are many and varied. They include protecting against UV-induced free radical damage and inflammation, as well as modulating cutaneous immune function. In this review, the author discusses the specific nutrients that can provide protoprotection, which are widely available in supplemental form to both aesthetic practitioners and patients alike.

Effects of Small Increases in Corticosterone Levels on Morphology, Immune Function, and Feather Development

Stressors encountered during avian development may affect an individual's phenotype, including immunocompetence, growth, and feather quality. We examined effects of simulated chronic low-level stress on American kestrel (Falco sparverius) nestlings. Continuous release of corticosterone, a hormone involved in the stress response, can model chronic stress in birds. We implanted 13-d-old males with either corticosterone-filled implants or shams and measured their growth, immune function, and feather coloration. We found no significant differences between groups at the end of the weeklong exposure period in morphometrics (mass, tarsus, wing length, and asymmetry), immunocompetence (cutaneous immunity, heterophil/lymphocyte ratio, and humoral immunity), or feather coloration. One week subsequent to implant removal, however, differences were detected. Sham-implanted birds had significantly longer wings and a reduced level of cutaneous immune function compared with those of birds given corticosterone-filled implants. Therefore, increases of only 2 ng/mL in basal corticosterone titer can have small but measurable effects on subsequent avian development.

1,25‐dihydroxy vitamin D3 regulates cutaneous innate immune function

Hormonally active vitamin D3 – 1,25‐dihydroxyvitamin D3 (1,25D3) – acts as a signalling molecule in cutaneous immunity. In this study we investigated if Toll‐like‐receptor (TLR) function and antimicrobial peptide (AMP) expression are controlled by 1,25D3 in keratinocytes. The AMP cathelicidin and TLR cofactor CD14 were known to be induced by 1,25D3, and analysis of TLR2 expression revealed this also was increased by 1,25D3. Topical 1,25D3 application to human skin confirmed these results, showing increased cathelicidin, CD14 and TLR2 by immunostaining. Furthermore, the presence of 1,25D3 enabled human keratinocytes to respond to Malp2 (a TLR2/6 ligand) with increased cathelicidin production which was inhibited by neutralizing antibody to TLR2. 1,25D3 also increased the ability of keratinocytes to kill Staphylococcus aureus. Interestingly, keratinocytes surrounding human skin wounds increased expression of CD14 and showed a previously known increase in cathelicidin AMP. Thus, we hypothesized that 1,25D3 was also a signalling molecule during skin injury. Supporting this, we found that CYP27B1, the enzyme that converts 25‐hydroxy vitamin D3 (25D3) to active 1,25D3, was significantly increased in wounds and induced in response to factors in the wound micromilieu such as TGFβ1 or TLR stimulation. Blocking the vitamin D receptor, inhibiting CYP27B1 enzymatic activity, or limiting 25D3 in culture each prevented TGFβ1 from inducing cathelicidin, CD14 or TLR2. Furthermore, mice deficient in CYP27B1 failed to increase CD14 in vivo following injury. Thus, this investigation demonstrates how injury initiates the innate immune response; 25D3 is activated to 1,25D3 by enzymatic conversion, a process triggered by microbial products or host factors such as TGFβ1. The increase in 1,25D3 then directly increases cathelicidin release and enables responsiveness to microbial products through induction of TLR cofactor CD14.

Neurotrophins Act as Neuroendocrine Regulators of Skin Homeostasis in Health and Disease

Neurotrophins regulate cutaneous innervation, act as growth and motility factors on structural skin cells such as keratinocytes and fibroblasts, modulate cutaneous immune function and even serve as stress mediators in skin biology. The multilayered neurotrophin interaction with skin biology through high affinity specific tyrosinekinase receptors and the Janus-faced p75 receptor, which depending on ligand and co-receptor expression can serve as a low-affinity pan-neurotrophin receptor or a high affinity proneurotrophin receptor, guaranties this neuroendocrine peptide family a central position in the control of skin homeostasis in health and disease. It is a challenging task for future research efforts to integrate our knowledge on differential neurotrophin expression patterns and signaling pathways into complex concepts of neuroendocrine tissue remodeling and pathogenetic processes. In addition, we need to improve our understanding of the role of neurotrophin processing enzymes, associated co-receptors and intracellular adaptor molecules in specific cutaneous cell populations to design precise interaction tools for research and treatment. Such tools will allow us to utilize this ancient growth factor family in the management of neurotrophin responsive pathogenetic pathways and cutaneous diseases such as neurogenic inflammation, peripheral nerve degeneration, wound healing, atopic dermatitis or psoriasis.

Neuropeptide (Calcitonin Gene-Related Peptide) Induction of Nitric Oxide in Human Keratinocytes in vitro

Nitric oxide (NO) is an important signaling molecule in both the central nervous system and the periphery, where it is involved in neurotransmission, vascular and bronchial tone, inflammation, and cutaneous immune function. More recently, NO has been implicated in intracellular signaling and may have a role in cellular differentiation, cytokine expression, and apoptosis. The experiments described herein examined the effect of calcitonin gene-related protein (CGRP), a cutaneous nerve neuropeptide, on NO production in human keratinocytes in vitro. CGRP stimulated two distinct increases in NO production: one within 30 minutes and a second at 24 hours. CGRP stimulated a modest increase in inducible nitric oxide synthase (iNOS) at 3-6 hours. Experimental evidence suggested that CGRP stimulated both constitutive NOS activity and generation of NO via nitrosothiol degradation within the first hour. Production of NO was paralleled by a decrease in nitrosothiol levels for 2 hour, suggesting that immediate NO release may originate from pre-existing stores. Nitrosothiols are ubiquitous molecules that comprise an important NO pool and have intracellular regulatory roles, particularly linked to oxidative stress. The present data indicate that, in addition to its known cAMP signaling pathway, CGRP may act to regulate keratinocyte biology through intracellular NO by modulation of S-nitrosothiol stores and stimulation of NOS activity.

Skin-Derived Dendritic Cells Induce Potent CD8 + T Cell Immunity in Recombinant Lentivector-Mediated Genetic Immunization

The skin contains readily accessible dendritic cells (DCs) with potent antigen presentation function and functional plasticity enabling the integration of antigen specificity with environmentally responsive immune control. Recent studies challenge the established paradigm of cutaneous immune function by suggesting that lymph node-resident DCs, rather than skin-derived DCs (sDCs), are responsible for eliciting T cell immunity against cutaneous pathogens including viral vectors. We show that cutaneous delivery of lentivirus results in direct transfection of sDCs and potent and prolonged antigen presentation. Further, sDCs are the predominant antigen-presenting cells for the induction of potent and durable CD8(+) T cell immunity. These results support the classical paradigm of cutaneous immune function and suggest that antigen presentation by sDCs contributes to the high potency of lentivector-mediated genetic immunization.

Vaccinia Virus Induces Strong Immunoregulatory Cytokine Production in Healthy Human Epidermal Keratinocytes: a Novel Strategy for Immune Evasion

Iatrogenic cutaneous infection with vaccinia virus (VV) and naturally occurring systemic infection with variola virus both lead to the characteristic skin "pox" lesions. Despite significant medical experience with both viruses, surprisingly little is understood about the interactions between these poxviruses and healthy resident skin cells. In recent years, it has become clear that skin plays an essential role in modulating both innate and adaptive immune responses, in part by producing and responding to a variety of cytokines and chemokines upon stimulation. Antagonists of many of these compounds are encoded in poxvirus genomes. Infection of skin cells with poxvirus may lead to a unique pattern of cytokine and chemokine production that might alter the cutaneous immune surveillance function. In this study, we infected primary cultures of human skin cells with VV and monitored antigen expression, virus replication, and cytokine production from the infected cells. While T cells, Langerhans cells, and dermal dendritic cells were infected abortively, keratinocytes, dermal fibroblasts, and dermal microvascular endothelial cells (HMVEC-d) all supported the complete virus life cycle. In contrast to the robust viral replication in fibroblasts and HMVEC-d, only limited viral replication was observed in keratinocytes. Importantly, VV infection of keratinocytes led to up-regulation of immunoregulatory and Th2 cytokines, including transforming growth factor beta, interleukin-10 (IL-10), and IL-13. We propose that the rapid induction of keratinocyte Th2 and immunoregulatory cytokines represents a poxvirus strategy to evade immune surveillance, and the limited viral multiplication in keratinocytes may be a protective mechanism to help the immune system "win the race."

Corticosterone suppresses cutaneous immune function in temperate but not tropical House Sparrows, Passer domesticus

Levels of corticosterone (CORT), the primary avian stress hormone, tend to vary over space and time in passerines, but why this is so remains unclear. One reason may be differential need for immune defense. Typically, sustained high levels of CORT suppress immune activity in vertebrates. Thus, animals living where parasite threats are high might maintain low levels of CORT and mount weak CORT stress responses to ensure that their immune defenses are in a high state of readiness at all times. Here, we addressed this hypothesis by comparing CORT levels in two populations of House Sparrows ( Passer domesticus), one from the tropics (Colon, Panama) where parasite threats are high and one from the North-temperate zone (New Jersey, USA) where they are lower. Indeed, we found that House Sparrows from Panama had lower baseline and stress-induced CORT levels than House Sparrows from New Jersey. To more directly test our hypothesis, we artificially elevated CORT (via implant) in both populations of birds, expecting that cutaneous immune activity (induced by phytohemagglutinin (PHA)) would be suppressed as it is in most vertebrates studied to date. Surprisingly, we found that CORT implants did not affect immune function in Panamanian sparrows, while immune function in (non-breeding) New Jersey sparrows was suppressed. This suggests that Panamanian House Sparrows may be immunologically insensitive to CORT, in addition to maintaining low baseline and stress-induced levels of this hormone. We propose that other animals living where disease threats are high may use CORT in a similar way.

Pituitary adenylate cyclase-activating polypeptide inhibits cutaneous immune function.

Epidermal nerves are closely associated with Langerhans cells (LC) and may be able to release factors, such as calcitonin gene-related peptide and epinephrine, that affect LC function. LC and the LC-like cell line XS106 express mRNA for the pituitary adenylate cyclase-activating polypeptide (PACAP) receptors VPAC1 and VPAC2. We examined whether PACAP regulates cutaneous immunity. Intradermal administration of PACAP prior to application of a contact sensitizer at the injected site inhibited the induction of contact hypersensitivity. Pretreatment of murine epidermal cells enriched for LC content (approximately 12% LC) with PACAP inhibited their ability to elicit delayed-type hypersensitivity in previously immunized mice. In vitro, PACAP suppressed the ability of both murine epidermal cells and highly purified LC (approximately 95%) to present antigen to a T cell clone and hybridoma. Furthermore, in LC and the XS106 cell line, PACAP inhibited the LPS/GM-CSF-induced stimulation of IL-1beta secretion and augmented IL-10 production. PACAP also down-regulated CD86 expression in LPS/GM-CSF-stimulated XS106 cells. The immunosuppressive effects of PACAP may be due to modulation of cytokine production and CD86 expression.

Altered cutaneous immune parameters in transgenic mice overexpressing viral IL-10 in the epidermis.

IL-10 is a pleiotropic cytokine that inhibits several immune parameters, including Th1 cell-mediated immune responses, antigen presentation, and antigen-specific T cell proliferation. Recent data implicate IL-10 as a mediator of suppression of cell-mediated immunity induced by exposure to UVB radiation (280-320 nm). To investigate the effects of IL-10 on the cutaneous immune system, we engineered transgenic mice that overexpress viral IL-10 (vIL-10) in the epidermis. vIL-10 transgenic mice demonstrated a reduced number of I-A(+) epidermal and dermal cells and fewer I-A(+) hapten-bearing cells in regional lymph nodes after hapten painting of the skin. Reduced CD80 and CD86 expression by I-A(+) epidermal cells was also observed. vIL-10 transgenic mice demonstrated a smaller delayed-type hypersensitivity response to allogeneic cells upon challenge but had normal contact hypersensitivity to an epicutaneously applied hapten. Fresh epidermal cells from vIL-10 transgenic mice showed a decreased ability to stimulate allogeneic T cell proliferation, as did splenocytes. Additionally, chronic exposure of mice to UVB radiation led to the development of fewer skin tumors in vIL-10 mice than in WT controls, and vIL-10 transgenic mice had increased splenic NK cell activity against YAC-1targets. These findings support the concept that IL-10 is an important regulator of cutaneous immune function.

Altered cutaneous immune parameters in transgenic mice overexpressing viral IL-10 in the epidermis

IL-10 is a pleiotropic cytokine that inhibits several immune parameters, including Th1 cell-mediated immune responses, antigen presentation, and antigen-specific T cell proliferation. Recent data implicate IL-10 as a mediator of suppression of cell-mediated immunity induced by exposure to UVB radiation (280-320 nm). To investigate the effects of IL-10 on the cutaneous immune system, we engineered transgenic mice that overexpress viral IL-10 (vIL-10) in the epidermis. vIL-10 transgenic mice demonstrated a reduced number of I-A(+) epidermal and dermal cells and fewer I-A(+) hapten-bearing cells in regional lymph nodes after hapten painting of the skin. Reduced CD80 and CD86 expression by I-A(+) epidermal cells was also observed. vIL-10 transgenic mice demonstrated a smaller delayed-type hypersensitivity response to allogeneic cells upon challenge but had normal contact hypersensitivity to an epicutaneously applied hapten. Fresh epidermal cells from vIL-10 transgenic mice showed a decreased ability to stimulate allogeneic T cell proliferation, as did splenocytes. Additionally, chronic exposure of mice to UVB radiation led to the development of fewer skin tumors in vIL-10 mice than in WT controls, and vIL-10 transgenic mice had increased splenic NK cell activity against YAC-1targets. These findings support the concept that IL-10 is an important regulator of cutaneous immune function.

Tumour necrosis factor-alpha-induced migration of human Langerhans cells: the influence of ageing.

Langerhans cells (LCs) play essential roles in the initiation and regulation of cutaneous immune responses mediated through their successful migration from the epidermis to draining lymph nodes while carrying antigen. Tumour necrosis factor (TNF)-alpha, a keratinocyte-derived cytokine, has recently been shown to play an important role in the mobilization of LCs from human epidermis. Although it is known that with age the immune system changes, the influence of increasing age on the function of human LCs has not been defined clearly. To examine the influence of age on the ability of TNF-alpha to induce LC migration. Ten elderly (six men, four women; mean age 76 years, range 72-79) and 10 young (six men, four women; mean age 23 years, range 18-35) volunteers received intradermal injections of 200 U of human recombinant TNF-alpha diluted in sterile saline, and control injections of sterile saline alone, at each of two paired sites identified on photoprotected buttock skin. Two hours later, paired injection sites were excised by punch biopsy. One set of paired biopsies was processed for assessment of the frequency and morphology of epidermal LCs, following preparation of epidermal sheets and immunofluorescence staining for the LC marker CD1a. The remaining paired biopsies were processed in formalin and the inflammatory response to TNF-alpha was assessed by standard histological examination. Mean +/- SEM baseline values for LC frequency within epidermal sheets were significantly different between young (1156.3 +/- 38.5 cells mm(-2)) and elderly subjects (835.7 +/- 48.2 cells mm(-2); P < 0.01). Intradermal injections of 200 U of TNF-alpha caused a significant reduction in the frequency of LCs in both elderly and young subjects (P < 0.01). However, the extent of TNF-alpha-induced LC migration was substantially different between the two groups, with a mean 9% reduction in LC frequency in elderly volunteers compared with a mean 23% decrease in young subjects. Exposure to TNF-alpha was associated with a perivascular polymorphonuclear infiltrate at 2 h in all young subjects; in contrast, only 50% of the elderly individuals showed evidence of such a response. There are significant differences between young and old skin with respect to both resting LC numbers and their response to TNF-alpha. These age-related changes in LC frequency and function may contribute to the altered cutaneous immune function observed in the elderly.

Regulation of epidermal Langerhans cell migration by lactoferrin.

Lactoferrin (LF) is a member of the transferrin family of iron-binding glycoproteins to which several anti-inflammatory functions have been ascribed. LF has been shown to down-regulate expression of the pro-inflammatory cytokine tumour necrosis factor-alpha (TNF-alpha), although the possibility has been raised that the activity of LF in this regard was indirect and secondary to its ability to bind to and inactivate the bacterial lipopolysaccharide (LPS) used to induce cytokine production. However, the identification of putative membrane receptors for LF raises the possibility that the interaction of LF with its receptor may be one important route through which this protein exerts anti-inflammatory activity. In the present investigations the biological properties of LF have been examined in a model of cutaneous immune function where the allergen-induced migration of epidermal Langerhans cells (LC) from the skin and their subsequent accumulation as dendritic cells (DC) in skin-draining lymph nodes are known to be dependent upon the de novo synthesis of TNF-alpha, but independent of exogenous LPS. Consistent with the protein having direct anti-inflammatory properties, it was found that the intradermal injection of recombinant murine LF (either iron-saturated or iron-depleted LF) inhibited significantly allergen (oxazolone) -induced LC migration and DC accumulation. That these inhibitory effects were secondary to the inhibition of local TNF-alpha synthesis was suggested by the findings that first, LF was unable to inhibit LC migration induced by intradermal injection of TNF-alpha itself, and second, that migration stimulated by local administration of another epidermal cytokine, interleukin 1beta, which is also dependent upon TNF-alpha production, was impaired significantly by prior treatment with LF. Finally, immunohistochemical analyses demonstrated the presence of LF in skin, associated primarily with keratinocytes. Collectively these data support the possession by LF of direct immunomodulatory and/or anti-inflammatory activity, probably associated in this case with inhibition of cytokine production. Furthermore, the results suggest that as a constituent of normal skin, LF may play a role in homeostatic regulation of cutaneous immune function.

Stress-induced enhancement of cell-mediated immunity.

We have demonstrated that acute stress induces a large-magnitude, rapid, and reversible redistribution of leukocytes from the blood to other compartments within the body. These changes in leukocyte distribution are mediated by adrenal stress hormones. Because the skin is one of the target organs of a stress-induced redistribution of leukocyes, we hypothesized that such a leukocyte redistribution could be one of the factors by which acute stress may enhance cutaneous immune function. This hypothesis was tested by examining the effects of acute stress on cutaneous delayed-type hypersensitivity (DTH). DTH reactions are antigen-specific, cell-mediated immune responses that, depending on the antigen involved, mediate beneficial (resistance to viruses, bacteria, and fungi) or harmful (allergic dermatitis, autoimmunity) aspects of immune function. DTH was induced by challenging the pinnae of previously sensitized rats with 2,4-dinitro-1-fluorobenzene (DNFB). Experiments showed that acute stress administered immediately before the introduction of an antigenic challenge significantly enhances a cutaneous DTH response. In contrast, chronic stress suppresses cutaneous DTH. These results demonstrate a bidirectional relationship between stress and immune function, such that acute stress enhances, while chronic stress suppresses, an important class of immune responses in vivo. They also suggest that stress-induced alterations in lymphocyte redeployment within the body may play an important role in mediating these bidirectional effects of stress on cell-mediated immunity.

In vitro immunosuppressive effects of methotrexate and azathioprine on Langerhans cells.

The long-term use of immunosuppressive agents may cause profound suppression of the cutaneous immune function. Because epidermal Langerhans cells (LC) play an important role in the cutaneous immune system, they could be the target of immunosuppressants. Since little information is available about the direct immunosuppressive effects of methotrexate (MTX) and azathioprine (AZA) on LC, we studied the viability and immunostimulatory effects of purified LC after pulsation with MTX or AZA at various concentrations. Both MTX and AZA started to suppress the mixed LC-T lymphocyte reaction (MLCLR) significantly at a concentration of 1 microgram/ml. However, the suppressive effect of MTX was not dose-dependent; no further suppression was seen up to a concentration of 1 mg/ml. MTX showed no inhibitory effect on the viability of LC even at concentrations as high as 1 mg/ml. In contrast, the suppressive effect of AZA on the MLCLR was dose-dependent and AZA at a concentration of 500 micrograms/ml or higher markedly decreased the viability of LC. Our study confirmed the immunosuppressive effect of MTX and AZA on epidermal LC. In comparison to MTX, AZA at pharmacological levels showed stronger inhibitory effects on alloantigen-presenting capacity of human epidermal LC and was more cytotoxic to LC. In the therapeutic range, however, MTX and AZA probably have no direct effect on epidermal LC. Our study supports the notion that long-term immunosuppressants deplete cutaneous LC by bone marrow suppression.

Effects of near-infrared radiation on the epidermal proliferation and cutaneous immune function in mice.

While ultraviolet radiation alters various cutaneous cell functions, little is known about the photobiological effects of infrared radiation (IR) on the skin except its local thermal effect. This study demonstrated that single exposure of mouse skin to near IR (0.7-1.3 microns) reversibly suppressed the proliferating activity of the epidermis, the density of Langerhans cells, and the ability of skin to induce contact hypersensitivity reaction. During the exposure, the ear surface temperature was elevated from a mean of 27 to 31.2 degrees C. The results suggest that near IR can modulate the epidermal proliferation and part of the skin immune system, with a mild thermal effect.

Experimental Photoaging in C3H/HeN, C3H/HeJ, and Balb/c Mice: Comparison of Changes in Extracellular Matrix Components and Mast Cell Numbers

Chronic exposure of human or murine skin to ultraviolet B (UVB) radiation alters dermal extracellular matrix composition and increases the number of mast cells and inflammatory cells. Experiments were designed to test the possible role of UVB-induced tumor necrosis factor-alpha in these photoaging changes based on reports that C3H/HeN, but not C3H/HeJ or Balb/c mice, produce excess TNF-alpha in response to UVB exposure. Pigmented C3H/HeN and C3H/HeJ strains were exposed to a total of 75 J/cm2 of UVB radiation, and unpigmented Balb/c mice were exposed to 19 J/cm2. The UVB-induced increases in collagen, glycosaminoglycans, and neutrophil number were similar or the same in all three strains. The elastin increase was greater in C3H/HeJ than in C3H/HeN mice. The most striking difference between the strains was a 7.7-fold UVB-induced increase in mast cells in C3H/HeN mice compared to no increase in irradiated C3H/HeJ mice and a 2.3-fold increase in Balb/c mice. These results suggest that excess TNF-alpha (or other mediator) produced in C3H/HeN skin (but not C3H/HeJ skin) in response to UVB exposure is involved in the mast cell increase and partial inhibition of elastin increase, but that neither these mediators nor mast cell products are important mediators for the chronic UVB-induced increases in neutrophils, glycosaminoglycans, and collagen. When a possible source of the excess TNF-alpha was investigated, it was found that isolated epidermal cells from all three strains produced increases in TNF-alpha in response to UVB radiation. These results, as well as the previous results showing differences between these strains in UVB-induced effects on cutaneous immune function, are consistent with a model in which UVB-induced mediators from the epidermis stimulate another cell type to produce excess TNF-alpha (and other mediators) in the C3H/HeN but not C3H/HeJ or Balb/c mice.