Influence of chemical reactivity of urushiol-type haptens on sensitization and the induction of tolerance.

Contact sensitization to components of the urushiol oils of poison oak and poison ivy appears to require covalent bond formation between the o-quinones derived from urushiol catechols and nucleophilic groups on proteins. Previous studies using a murine delayed hypersensitivity model demonstrated that 5-methyl-3-pentadecylcatechol (5-Me-PDC) is an epicutaneous tolerogen to the parent compound and a weak sensitizer to itself. To investigate further the structural requirements for sensitization vs suppression, 5,6-dimethyl-3-pentadecylcatechol (5,6-di-Me-PDC) and 4,5,6-trimethylpentadecylcatechol (4,5,6-tri-Me-PDC) were synthesized. The former compound is blocked at both preferred sites for covalent bond formation and the latter is completely blocked towards conjugate addition reactions. These compounds were tested for sensitizing and suppressive ability. Epicutaneous application of both analogs suppressed subsequent induction of sensitization to 3-pentadecylcatechol (PDC) and 3-heptadecylcatechol (HDC). Lymph node cells from animals treated with 5,6-di-Me-PDC could transfer suppression. The dimethyl analog, 5,6-di-Me-PDC, but not the trimethyl analog also exhibited weak sensitizing capacity. The urushiol analogs 5-pentadecylresorcinol (PDR) and 3-heptadecylveratrole (HDV) which cannot form o-quinones were found to be ineffective sensitizers as well. HDV in addition produced no blastogenesis in draining lymph nodes whereas lymph node cell proliferation induced by 4,5,6-tri-Me-PDC followed the same kinetics as previously observed for HDC. PDR elicited weak proliferation with a different time course. These and previous studies indicate that blocking the C5-position on the catechol ring favors the induction of suppression, although some sensitizing capacity may be retained. Covalent bond formation may not be necessary for the induction of active suppressor cell populations.