Application of arachidonic acid (AA) (0.1-4 mg) to the ears of mice produces immediate vasodilatation and erythema (5 min) followed by the abrupt development of edema which is maximal at 40-60 min. The onset of edema coincides with extravasation of protein and leukocytes. After 1 h, the edema begins to wane rapidly and the inflammatory cells leave the tissue so that by 6 h the ears have returned to near normal except for residual erythema. During the period 6-48 h, AA-treated ears show a greatly diminished response with respect to edema and cell infiltrate when AA is applied a second time. Inhibitor studies show that the inflammatory response is due to formation of AA metabolites via both the cyclooxygenase and lipoxygenase pathways. Under appropriate conditions, AA-induced ear edema can be used as a model to screen for compounds showing in vivo lipoxygenase inhibitory activity. Although relatively large doses of AA were applied topically, there was only a modest stimulation of epidermal DNA synthesis and mitotic index with no consequent hyperplasia. Although arachidonic acid is capable of eliciting most aspects of an inflammatory response, the reaction is abrupt in onset and of short duration. Additional factors appear to be required to produce a prolonged inflammatory response with associated tissue destruction, or inflammatory cell activation and immobilization in situ.

12-0-Tetradecanoylphorbol acetate (TPA) applied to mouse ears rapidly induces an edema which is maximal by 6 hr but has substantially waned by 24 hr. (This is in contrast to many inflammatory agents that cause a prolonged edema lasting many days.) Reapplication of TPA at 16-24 hr will not provoke a second edematous response although increased erythema is evident. Arachidonic acid (AA) applied to mouse ears (4 mg) provokes an even more rapid edema which is maximal at 1 hr and has substantially waned by 6 hr. Reapplication of AA at 3-24 hr also will not provoke a second edematous response although, again, increased erythema does result. Pretreatment of ears with AA results in inhibition of the edema response to subsequent application of TPA, and TPA pretreatment moderately inhibits a subsequent response to AA. TPA-induced edema can be delayed by agents such as naproxen, an inhibitor of AA cyclooxygenase. In contrast, AA-induced edema is inhibited only by agents, such as phenidone, that inhibit both cyclooxygenase and lipoxygenase. The data suggest that the edemas result from interaction of the products of the cyclooxygenase and lipoxygenase pathways of AA metabolism. The lack of secondary edema response appears to be related to the inability of TPA or AA to reinduce vascular permeability. The effect is specific to AA and TPA; responses to xylene or anthralin are unaffected by TPA or AA pretreatment. It is postulated that the tachyphylactic effects observed involve lipoxygenase metabolites of AA.

No abstract

The intradermal injection of 140 micrograms of Propionibacterium acnes (CN 6134) into the ears of female Sprague-Dawley rats produced a chronic inflammation with formation of acneiform lesions. Inflammation was characterized by more than a doubling of ear thickness at 24 h and a peak of 3-4 times control levels at day 21. At 42 days post injection ears were still 3 times normal thickness. Histologically there was early polymorph accumulation giving way to macrophages and lymphocytes by day 7. Pilosebaceous follicles overlying the inflamed area lost their sebaceous glands and became hyperplastic cords of cells that grew down and encapsulated inflammatory loci. By day 9 many of these follicles had become secondary comedones. Three isolates of P. acnes from inflammatory acne lesions and 4 of 5 isolates from non-acne patients produced results similar to that of the strain CN 6134. In these cases the number of histologically evident secondary comedones was correlated with ear thickness. In contrast, samples of Streptococcus lactis, Escherichia coli B, and Staphylococcus epidermidis failed to produce this combination of chronic inflammation and high lesion count. Benzoyl peroxide, tetracycline, erythromycin, phenidone, naproxen, and cis and trans retinoic acid were inactive as inhibitors of P. acnes CN 6134-induced ear thickening. The corticosteroid fluocinolone acetonide produced dramatic suppression of inflammation, but upon cessation of treatment the ears returned to inflamed levels. The specificity for P. acnes, the formation of acneiform lesions, and the recalcitrance of the inflammation suggest our model is indeed relevant to acne.

The ability of strains and fractions of killed propionibacteria suspensions to produce chronic rat ear inflammation after intradermal injection of 70-micrograms aliquots was highly correlated with production of splenomegaly in the mouse after i.p. injection of 1.4 mg Propionibacterium acnes strains CN 6134, VPI 0009, ATCC 11828, and UCLA SC and N1 produced a 2- to 3-fold increase in rat ear thickness and a 5- to 7-fold increase in mouse spleen weight 15 days post injection. In contrast P. granulosum CN 5888, P. acnes UCLA 6S and periodated, acetylated, or 12-h cultures of VPI 0009 were inactive or weakly active as stimulators of chronic ear inflammation and splenomegaly. Active strains produced in the rat ear a transepidermal elimination response characterized by follicular encapsulation and the formation of secondary comedones. These effects correlated with persistence of phagocytized bacteria within macrophages. Furthermore, when rats were first immunized and then challenged with active strains of P. acnes, an increased sensitivity to low doses of P. acnes and a chronic exacerbation of inflammation was observed.