The significance of CYP11A1 expression in skin physiology and pathology.

CYP11A1, a member of the cytochrome P450 family, plays several key roles in the human body. It catalyzes the first and rate-limiting step in steroidogenesis, converting cholesterol to pregnenolone. Aside from the classical steroidogenic tissues such as the adrenals, gonads and placenta, CYP11A1 has also been found in the brain, gastrointestinal tract, immune systems, and finally the skin. CYP11A1 activity in the skin is regulated predominately by StAR protein and hence cholesterol levels in the mitochondria. However, UVB, UVC, CRH, ACTH, cAMP, and cytokines IL-1, IL-6 and TNFα can also regulate its expression and activity. Indeed, CYP11A1 plays several critical roles in the skin through its initiation of local steroidogenesis and specific metabolism of vitamin D, lumisterol, and 7-dehydrocholesterol. Products of these pathways regulate the protective barrier and skin immune functions in a context-dependent fashion through interactions with a number of receptors. Disturbances in CYP11A1 activity can lead to skin pathology.

The clinical course of actinic keratosis correlates with underlying molecular mechanisms.

BACKGROUND: Actinic keratoses (AKs) are common premalignant skin lesions triggered by excessive ultraviolet exposure. The majority of AKs regress or persist, but some progress to squamous cell carcinomas. Biomarkers associated with their persistence, progression and regression have not been characterized. OBJECTIVES: We performed skin biopsies in patients with extensive actinic damage to identify biomarkers that correlate with clinical progression and regression of AKs. METHODS: This was an observational study of a cohort of patients with extensive actinic damage. AKs were mapped on a clear plastic template in 26 patients at months 3, 6, 9 and 11. Biopsies were taken from randomly selected, predetermined AKs and were evaluated for p53, E-cadherin, Snail, Slug and Twist. The study is registered at Clinicaltrials.gov: NCT00027976. RESULTS: p53 exhibited greater expression in clinically apparent AKs (histological score 2·89 ± 1·45) than in regressed AKs (0·75 ± 0·96); P < 0·01. There was also significantly less membrane E-cadherin, the lack of which is a marker of epithelial-mesenchymal transition, in clinically apparent AKs (1·89 ± 1·81) than in sun-exposed skin (3·07 ± 1·75); P < 0·005. The E-cadherin transcription repressors Snail, Slug and Twist were increased in AKs compared with sun-exposed skin. A limitation of the study is that measurement of histological biomarkers was not a primary end point. In addition, patients were allowed to apply sunscreens. CONCLUSIONS: At the molecular level, loss of E-cadherin and an increase in p53 are linked to the dynamic interplay between the persistence, progression and regression of AKs. What's already known about this topic? Actinic keratoses (AKs) are common dysplastic epidermal lesions that result from chronic and excessive ultraviolet exposure. Biomarkers associated with progression and regression of AK have not been characterized. What does this study add? Decreased E-cadherin and increased p53, Snail, Slug and Twist (E-cadherin transcription factors) were associated with progression from AK to nonmelanoma skin cancer. What is the translational message? Strategies targeting these molecules may be effective in reversing rising skin cancer rates. E-cadherin, p53, Snail, Slug and Twist are potential biomarkers that may be used to assess the efficacy of existing chemopreventive agents.

Topical application of delphinidin reduces psoriasiform lesions in the flaky skin mouse model by inducing epidermal differentiation and inhibiting inflammation.

BACKGROUND: Psoriasis is a chronic inflammatory skin disease characterized by hyperproliferation and aberrant keratinocyte differentiation. We have shown that treatment of reconstituted human skin with delphinidin, an anthocyanidin, present in pigmented fruits and vegetables, increased the expression and processing of caspase-14, which is involved in cornification. Delphinidin also increases the expression of epidermal differentiation marker proteins. OBJECTIVES: To determine whether topical application of delphinidin can modulate pathological markers of psoriasiform lesions in flaky skin mice and if this is associated with increased epidermal differentiation and a reduction in proliferation and inflammation. METHODS: Five-week-old female homozygous flaky skin mice (fsn/fsn) were treated topically with delphinidin (0·5 mg cm(-2) and 1 mg cm(-2) skin areas, respectively), five times a week, up to 14 weeks of age. RESULTS: Treatment of flaky skin mice with delphinidin resulted in a reduction in (i) pathological markers of psoriasiform lesions; (ii) infiltration of inflammatory cells; and (iii) mRNA and protein expression of inflammatory cytokines. Delphinidin treatment also increased the expression and processing of caspase-14, and expression of filaggrin, loricrin, keratin-1 and keratin-10. Furthermore, there was a decrease in the expression of markers for cell proliferation (proliferating cell nuclear antigen and keratin-14) and modulation of tight junction proteins (occludin and claudin-1). In addition, delphinidin treatment increased the expression of activator protein-1 transcription factor proteins (JunB, JunD, Fra1 and Fra2). CONCLUSIONS: Delphinidin could be a promising agent for treatment of psoriasis and other hyperproliferative skin disorders.

Visualizing CD4 T-cell migration into inflamed skin and its inhibition by CCR4/CCR10 blockades using in vivo imaging model.

BACKGROUND: Chemokines are critical mediators of T-cell homing into inflamed skin. The complex nature of this multicellular response makes it difficult to analyse mechanisms mediating the early responses in vivo. OBJECTIVES: To visualize directly T-cell homing into inflamed skin and its inhibition by blockades using a unique noninvasive confocal microscopy. MATERIALS AND METHODS: A mouse model of allergic contact dermatitis was used. T cells from oxazolone-sensitized and -challenged Balb/c mice were first analysed phenotypically in vitro. CD4 T cells were then labelled with a tracker dye and transferred into Balb/c-SCID mice. The recipient mice were challenged with oxazolone and CD4 T-cell homing into inflamed skin was visualized. RESULTS: T cells with the skin homing receptors CCR4 and CCR10 were increased in the affected skin and draining lymph nodes, and effectively attracted by their specific chemokines CCL17, CCL22 and CCL27 in vitro. Using in vivo imaging, T-cell migration into the inflamed skin was observed at 2 h after application, peaking at 12 h and continuing for 48 h. Simultaneous systemic administration of neutralizing antibodies against CCR4 ligands (CCL17 and CCL22) and CCR10 ligand (CCL27) led to a significant suppression of T-cell migration and skin inflammation. CONCLUSIONS: Our data indicate that these tissue-selective adhesion molecules and chemokine/receptor pathways act in concert to attract specialized T-cell populations to mediate cutaneous inflammation. The in vivo imaging technique can be applicable to other models of cutaneous diseases to help with better understanding of the pathogenesis and monitoring the therapeutic effects.

Protection against tetanus by needle-free inoculation of adenovirus-vectored nasal and epicutaneous vaccines.

The effectiveness of vaccination programs would be enhanced greatly through the availability of vaccines that can be administered simply and, preferably, painlessly without the need for timed booster injections. Tetanus is a prime example of a disease that is readily preventable by vaccination but remains a major threat to public health due to the problems associated with administration of the present vaccine. Here we show that a protective immune response against live Clostridium tetani infection in mice can be elicited by an adenovirus vector encoding the tetanus toxin C fragment when administered as a nasal or epicutaneous vaccine. The results suggest that these vaccination modalities would be effective needle-free alternatives. This is the first demonstration that absorption of a small number of vectored vaccines into the skin following topical application of a patch can provide protection against live bacteria in a disease setting.

The role of ICAM-1 molecule in the migration of Langerhans cells in the skin and regional lymph node.

ICAM-1 (CD54) plays an important role in the cell-cell interaction and migration of leukocytes. Previous studies have shown that ICAM-1 is involved in inflammatory reactions and that a defect in ICAM-1 gene inhibits allergic contact hypersensitivity. This study indicates that the migration of hapten presenting Langerhans cells into the regional lymph nodes was significantly reduced in ICAM-1-deficient mice compared to wild-type C57BL/6 mice. The reduced number of dendritic cells in regional lymph nodes did not result from abnormal migration of Langerhans cells into the skin of ICAM-1-deficient mice. The concentration and distribution of Langerhans cells in the naïve skin of ICAM-1-deficient mice was equal to that of wild-type mice. Following hapten sensitization, Langerhans cell migration out of the skin and recruitment of fresh Langerhans cells back to the epidermis was not affected in ICAM-1-deficient mice. Further experiments demonstrated that ICAM-1 deficiency on lymphatic endothelium rather than on dendritic cells was responsible for the reduced migration of Langerhans cells into draining lymph nodes. This study indicates that ICAM-1 regulates the migration of dendritic cells into regional lymph nodes but not into or out of the skin.

Host defense mechanisms in polyaromatic hydrocarbon carcinogenesis.

Polyaromatic hydrocarbons (PAHs) are chemicals that are widely employed to examine the complex mechanisms by which chemicals cause cancer. While it is clear that the tumors that carcinogenic PAHs produce elicit an immune response, the interplay between host immune defense mechanisms and earlier stages in the cutaneous carcinogenesis pathway has received little attention. Studies from our laboratories have shown that topical application of several different PAHs to mice results in the development of an antigen-specific cell-mediated immune response to them. The response is genetically determined and is mediated by CD8+ T cells. Development of a cell-mediated immune response is associated with resistance to dimethylbenz(a)anthracene tumorigenesis. These findings are consistent with the hypothesis that host defense mechanisms against PAHs help to protect individuals from the carcinogenic actions of these agents. This may form the basis for novel immunopreventive strategies for individuals at high risk for development of tumors produced by PAHs.

Green tea polyphenolic antioxidants and skin photoprotection (Review).

Green tea is consumed as a popular beverage worldwide particularly in Asian countries like China, Korea, Japan and India. It contains polyphenolic compounds also known as epicatechins, which are antioxidant in nature. Many laboratories have shown that topical treatment or oral consumption of green tea polyphenols inhibits chemical carcinogen- or ultraviolet radiation-induced skin tumorigenesis in different animal models. Studies have shown that green tea extract also possesses anti-inflammatory activity. These anti-inflammatory and anti-carcinogenic properties of green tea are due to their polyphenolic constituents present therein. The major and most chemopreventive constituent in green tea responsible for these biochemical or pharmacological effects is (-)-epigallocatechin-3-gallate (EGCG). Understanding the molecular mechanisms of these effects of green tea is a subject of investigation in many laboratories. Treatment of green tea polyphenols to skin has been shown to modulate the biochemical pathways involved in inflammatory responses, cell proliferation and responses of chemical tumor promoters as well as ultraviolet (UV) light-induced inflammatory markers of skin inflammation. Topical treatment with EGCG on mouse skin also results in prevention of UVB-induced immunosuppression, and oxidative stress. The protective effects of green tea treatment on human skin either topically or consumed orally against UV light-induced inflammatory or carcinogenic responses are not well understood. Based on documented extensive beneficial effects of green tea on mouse skin models and very little in human skin, many pharmaceutical and cosmetic companies are supplementing their skin care products with green tea extracts. Therefore, the focus of this communication is to review and analyze the photoprotective effects of green tea polyphenols to skin.

A phase III, randomized, double-blind, placebo-controlled study of peldesine (BCX-34) cream as topical therapy for cutaneous T-cell lymphoma.

BACKGROUND: The purine nucleoside phosphorylase inhibitor peldesine is a new agent being evaluated as a T-cell inhibitor. OBJECTIVE: We attempted to determine the efficacy of peldesine (BCX-34) in a 1% dermal cream formulation as a treatment for cutaneous T-cell lymphoma (CTCL). METHODS: Ninety patients with patch and plaque phase CTCL, histologically confirmed by a referee dermatopathologist, were enrolled in a randomized, double-blind, placebo-controlled study. BCX-34 dermal cream 1% or the vehicle cream (as a placebo control) was applied twice daily to the entire skin surface for up to 24 weeks. Efficacy of the topical therapy was assessed in terms of complete or partial (> or = 50%) clearing of patches and plaques. RESULTS: Of the 89 patients able to be examined, 43 received BCX-34 and 46 received the placebo vehicle cream. One patient withdrew early and was not analyzed. The two groups were well balanced for potential prognostic factors. A total of 28% (12/43) of the patients treated with BCX-34 showed a response, but 24% (11/46) of patients who received vehicle also responded (P =.677). CONCLUSION: Although BCX-34 dermal cream 1% was not significantly better than the control as therapy for patch and plaque-phase CTCL, this appears to be the first published placebo-controlled trial evaluating treatment for CTCL. Whether the vehicle cream has more than a placebo therapeutic effect is unclear. The relatively high (24%) placebo response rate should be kept in mind in assessing other treatments for early-stage CTCL.

Cutaneous photoprotection from ultraviolet injury by green tea polyphenols.

BACKGROUND: In animal models, extracts from green tea have been shown to be remarkably effective at reducing the severity of adverse human health effects of overexposure to ultraviolet (UV) radiation. Although sunscreens and other photoprotective measures have traditionally been used for this purpose, there is a need for additional measures and natural products are increasingly being explored for that purpose. OBJECTIVE: Our purpose was to evaluate the effect of polyphenols from green tea on parameters associated with acute UV injury. METHODS: Areas of skin of normal volunteers were treated with an extract of green tea or one of its constituents. Thirty minutes later, the treated sites were exposed to a 2 minimal erythema dose solar simulated radiation. UV-treated skin was examined clinically for UV-induced erythema, histologically for the presence of sunburn cells or Langerhans cell distributions, or biochemically for UV-induced DNA damage. RESULTS: Application of green tea extracts resulted in a dose-dependent inhibition of the erythema response evoked by UV radiation. The (-)-epigallocatechin-3-gallate (EGCG) and (-)-epicatechin-3-gallate (ECG) polyphenolic fractions were most efficient at inhibiting erythema, whereas (-)-epigallocatechin (EGC) and (-)-epicatechin (EC) had little effect. On histologic examination, skin treated with green tea extracts reduced the number of sunburn cells and protected epidermal Langerhans cells from UV damage. Green tea extracts also reduced the DNA damage that formed after UV radiation. CONCLUSION: Polyphenolic extracts of green tea are effective chemopreventive agents for many of the adverse effects of sunlight on human health and may thus serve as natural alternatives for photoprotection.

Green tea polyphenols: DNA photodamage and photoimmunology.

Green tea is a popular beverage consumed worldwide. The epicatechin derivatives, which are commonly called 'polyphenols', are the active ingredients in green tea and possess antioxidant, anti-inflammatory and anti-carcinogenic properties. Studies conducted by our group on human skin have demonstrated that green tea polyphenols (GTP) prevent ultraviolet (UV)-B-induced cyclobutane pyrimidine dimers (CPD), which are considered to be mediators of UVB-induced immune suppression and skin cancer induction. GTP treated human skin prevented penetration of UV radiation, which was demonstrated by the absence of immunostaining for CPD in the reticular dermis. The topical application of GTP or its most potent chemopreventive constituent (-)-epigallocatechin-3-gallate (EGCG) prior to exposure to UVB protects against UVB-induced local as well as systemic immune suppression in laboratory animals. Additionally, studies have shown that EGCG treatment of mouse skin inhibits UVB-induced infiltration of CD11b+ cells. CD11b is a cell surface marker for activated macrophages and neutrophils, which are associated with induction of UVB-induced suppression of contact hypersensitivity responses. EGCG treatment also results in reduction of the UVB-induced immunoregulatory cytokine interleukin (IL)-10 in skin as well as in draining lymph nodes, and an elevated amount of IL-12 in draining lymph nodes. These in vivo observations suggest that GTPs are photoprotective, and can be used as pharmacological agents for the prevention of solar UVB light-induced skin disorders associated with immune suppression and DNA damage.

Sensitization versus elicitation in allergic contact dermatitis: potential differences at cellular and molecular levels.

The induction and elicitation phases of allergic contact dermatitis are very complicated processes that differ in many respects. Many issues remain to be investigated, especially in the elicitation phase of this disease. Several key molecules involved in the development and maintenance of allergic contact dermatitis have already been defined, including a number of different cytokines and adhesion molecules. Attempts are being made to develop new, more specific pharmacologic and immunologic inhibitors of their activity. Because patients who see a doctor are already sensitized, further elucidation of the factors affecting the elicitation of allergic contact dermatitis will greatly contribute to the prevention and treatment of the disease.

Regulation of TNFalpha production and release in human and mouse keratinocytes and mouse skin after UV-B irradiation.

TNFalpha is a primary cytokine responsible for inflammatory and immunosuppressive responses in skin. After UV-B irradiation of cultured human keratinocytes, we found that TNFalpha was released into the media, as monitored by ELISA, and was bound to cells, as observed by immunofluorescence microscopy. The release of TNFalpha into cell culture supernatant during the 24 h after UV-B irradiation was augmented by the addition of IL-1alpha to the cells. Further, we found this secretion was unaffected by rapamycin, and therefore independent of FRAP DNA-protein kinase mediated signal transduction. However, UV-B also induced expression of membrane-bound TNFalpha, and this was dependent on FRAP signaling. In wild type mice, TNFalpha bound to skin increased immediately after irradiation, declined at 6 h, and then rose again at 12 h before falling by 24 h. This pattern of induction was confirmed by RT-PCR of TNFalpha mRNA message in cultured epidermal cells. Induction of membrane-bound TNFalpha was also found in c-fos gene knockout mice deficient in the AP-1 transcription factor, suggesting that, although AP-1 containing c-fos signaling is required for some UV responses, AP-1 containing c-fos is not required for this TNFalpha activation. However, in homozygous p53 knockout mice the basal level of TNFalpha bound to the epidermis was greatly elevated without UV irradiation. This level declined and remained constant following irradiation. This implies that p53 directly or indirectly represses TNFalpha gene expression and that modification of p53 mRNA stability or phosphorylation of p53 protein after UV may be responsible for TNFalpha induction in the membrane. Overexpression of the immunosuppressive cytokine TNFalpha in this locale may contribute to the carcinogen-susceptibility of p53 knockout mice.

Immunosuppressive effects of silicon phthalocyanine photodynamic therapy.

The purpose of this study was to determine if silicon phthalocyanine 4 (Pc 4), a second-generation photosensitizer being evaluated for the photodynamic therapy (PDT) of solid tumors, was immunosuppressive. Mice treated with Pc 4 PDT 3 days before dinitrofluorobenzene sensitization showed significant suppression of their cell-mediated immune response when compared to mice that were not exposed to PDT. The response was dose dependent, required both Pc 4 and light and occurred at a skin site remote from that exposed to the laser. The immunosuppression could not be reversed by in vivo pre-treatment of mice with antibodies to tumor necrosis factor-alpha or interleukin-10. These results provide evidence that induction of cell-mediated immunity is suppressed after Pc 4 PDT. Strategies that prevent PDT-mediated immunosuppression may therefore enhance the efficacy of this therapeutic modality.

Polyphenolic antioxidant (-)-epigallocatechin-3-gallate from green tea reduces UVB-induced inflammatory responses and infiltration of leukocytes in human skin.

Identification of natural products capable of affording protection against UVB radiation-induced inflammatory responses and generation of oxidative stress may have important human health implications. The UVB exposure-induced skin injury and oxidative stress has been associated with a variety of skin disease conditions including photoaging, inflammation and cancer. Tea is a popular beverage consumed worldwide. In several mouse skin models, topical application as well as oral consumption of green tea has been shown to afford protection against chemical and UVB-induced carcinogenesis and inflammatory responses. In the present study, we investigated in human skin, whether topical application of (-)-epigallocatechin-3-gallate (EGCG), the major polyphenolic constituent in green tea, inhibits UVB-induced infiltration of leukocytes (macrophage/neutrophils), a potential source of generation of reactive oxygen species (ROS), and generation of prostaglandin (PG) metabolites. Human subjects were UVB irradiated on sun-protected skin to four times their minimal erythema dosage (MED) and skin biopsies or keratomes were obtained either 24 h or 48 h later. We found that topical application of EGCG (3 mg/2.5 cm2) before UVB (4 MED) exposure to human skin significantly blocked UVB-induced infiltration of leukocytes and reduced myeloperoxidase activity. These infiltrating leukocytes are considered to be the major source of generation of ROS. In the same set of experiments we found that topical application of EGCG before UVB exposure decreased UVB-induced erythema. In additional experiments, we found that microsomes from EGCG pretreated human skin and exposed to UVB, compared to UVB exposure alone, produced significantly reduced PG metabolites, particularly PGE2. The PG metabolites play a critical role in free radical generation and skin tumor promotion in multistage skin carcinogenesis. Careful microscopic examination of skin sections, stained with hematoxylin and eosin, under higher magnification (x400) also revealed that EGCG pretreated and UVB-exposed human skin contained fewer dead cells in the epidermis with comparison to nonpretreated UVB-exposed skin. Taken together, our data demonstrate that EGCG has the potential to block the UVB-induced infiltration of leukocytes and the subsequent generation of ROS in human skin. This may explain the possible mechanism involved in anti-inflammatory effects of green tea. We suggest that EGCG may be useful as a topical agent for protection against UVB-induced ROS-associated inflammatory dermatoses, photoaging and photocarcinogenesis. Further studies are warranted in this direction.

Phthalocyanine 4 (Pc 4) photodynamic therapy of human OVCAR-3 tumor xenografts.

Photodynamic therapy (PDT) is a cancer treatment modality utilizing a photosensitizer, light and oxygen. Photodynamic therapy with Photofrin has been approved by the U.S. Food and Drug Administration for treatment of advanced esophageal and early lung cancer. Because of certain drawbacks associated with the use of Photofrin, there is a need to identify new photosensitizers for human use. The photosensitizer Pc 4 (HOSiPc-OSi[CH3]2[CH2]3N[CH3]2) has yielded promising PDT effects in many in vitro and in vivo systems. The aim of this study was to assess the usefulness of Pc 4 as a PDT photosensitizer for a human tumor grown as a xenograft in athymic nude mice. The ovarian epithelial carcinoma (OVCAR-3) was heterotransplanted subcutaneously in athymic nude mice. Sixty mice bearing OVCAR-3 tumors (approximately 80-130 mm3) were divided into six groups of 10 animals each, three for controls and three for treatment. The Pc 4 was given by tail vein injection, and 48 h later a 1 cm area encompassing the tumor was irradiated with light from a diode laser coupled to a fiberoptic terminating in a microlens (lambda = 672 nm, 150 J/cm2, 150 mW/cm2). Tumors of control animals receiving no treatment, light alone or Pc 4 alone continued to grow. Of animals receiving 0.4 mg/kg Pc 4 and light, one (10%) had a complete response and was cured (no regrowth up to 90 days post-PDT), while all others (90%) had a partial response and were delayed in regrowth. Of animals receiving 0.6 mg/kg Pc 4 and light, eight (80%) had a complete response, and two of these were cured. Of animals receiving 1.0 mg/kg Pc 4 and light, six (60%) had a complete response, and two of these were cured. In additional experiments, tumors from animals treated with Pc 4 (1 mg/kg) and light were removed 15, 30, 60 and 180 min post-PDT, and from these tumors DNA and protein were extracted. Agarose gel electrophoresis revealed the presence of apoptotic DNA fragmentation as early as 15 min post-PDT. Western blotting showed the cleavage of the 116 kDa native poly(ADP-ribose) polymerase (PARP) into fragments of approximately 90 kDa, another indication of apoptosis, and the presence of p21/WAF1/CIP1 (p21) in all PDT-treated tumors. These changes did not occur in control tumors. Pc 4 appears to be an effective photosensitizer for PDT of human tumors grown as xenografts in nude mice. Early apoptosis, as revealed by PARP cleavage, DNA fragmentation and p21 overexpression, may be responsible for the excellent Pc 4-PDT response. Clinical trials of Pc 4-PDT are warranted.

Susceptibility to the biological effects of polyaromatic hydrocarbons is influenced by genes of the major histocompatibility complex.

Polyaromatic hydrocarbons are ubiquitous environmental chemicals that are important mutagens and carcinogens. The purpose of this study was to determine whether genes within the major histocompatibility complex (MHC) influence their biological activities. Cell-mediated immunity to dimethylbenz(a)anthracene (DMBA) was investigated in congenic strains of mice. On three different backgrounds, H-2(k) and H-2(a) haplotype mice developed significantly greater contact-hypersensitivity responses to DMBA than H-2(b), H-2(d), and H-2(s) mice. In B10.A(R1) mice, which are Kk and Id, a vigorous contact-hypersensitivity response was present, indicating that the response was governed by class I, rather than class II, MHC genes. C3H/HeN (H-2(k)) and C3H.SW (H-2(s)) strains were also compared for the development of skin tumors and the persistence of DMBA-DNA adducts. When subjected to a DMBA initiation, phorbol 12-tetradecanoate 13-acetate (TPA)-promotion skin-tumorigenesis protocol, C3H/HeN mice, (which develop cell-mediated immunity to DMBA) were found to have significantly fewer tumors than C3H.SW mice (a strain that failed to develop a cell-mediated immune response to DMBA). DMBA-DNA adducts were removed more rapidly in C3H/HeN than in C3H.SW mice. The results indicate that genes within the MHC play an important role in several of the biological activities of carcinogenic polyaromatic hydrocarbons. The observations are consistent with the hypothesis that cell-mediated immunity to chemical carcinogens serves to protect individuals by removing mutant cells before they can evolve into clinically apparent neoplasms.