Draining lymph node cells isolated from mice 24 h following topical exposure to a variety of contact-sensitizing chemicals, including the dinitrobenzene derivatives, 2,4-dinitrochlorobenzene and 2,4-dinitrothiocyanobenzene, contained increased numbers of dendritic cells (DCs). The increase in frequency of DCs was time-dependent and preceded significant changes in either lymph node cellularity or lymph node cell pro-liferative activity. The degree of DC accumulation was also influenced by the chemical used and the concentration employed for sensitization. In the context of contact allergy, the biological relevance of this phenomenon to the induction of hapten-specific responses is indicated by the fact that relatively small numbers of DC-enriched fractions of lymph node cells (comprising approximately 70% DCs), but not unfractionated or DC-depleted populations, transferred sensitization to naive animals. Moreover, using the skin-sensitizing fluorochrome, fluorescein isothiocyanate, it was observed that 24 h following exposure the majority of lymph node cells bearing high concentrations of antigen were within the DC-rich fraction.

The influence of topical exposure to two sensitizing chemicals on draining lymph node cell proliferative responses in BALB/c mice has been examined. Conventional contact sensitization with 4-ethoxymethylene-2-phenyloxazol-5-one (oxazolone) has been shown to induce a rapid and systemic suppression of subsequent proliferative responses to topically applied chemical which can be adoptively transferred to recipient mice with immune lymph node cells. In contrast to some previous reports in which such suppression was found to be largely antigen-specific in nature, we report that, at least initially, the inhibition of lymphocyte proliferation induced by skin sensitization is hapten-non-specific. The relevance of this phenomenon to the regulation of contact sensitization is discussed.

We have investigated whether there exists a correlation between the induction of draining lymph node cell (LNC) proliferation in contact allergy and the accumulation of dendritic cells (DC) within such lymph nodes. CBA/Ca mice, which compared with mice of BALB/c strain, mount a more vigorous lymphocyte proliferative response following sensitization, also exhibited a more marked accumulation of DC in draining lymph nodes 24 h following skin painting. Moreover, studies with the skin-sensitizing fluorochromes fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC) revealed that DC-enriched fractions of draining LNC prepared from CBA/Ca mice contained a higher percentage of antigen-bearing cells than did those from BALB/c mice. A relationship between DC migration into lymph nodes and the magnitude of the induced LNC proliferative response was also indicated by experiments performed in BALB/c mice with a variety of contact allergens. It was observed that there was a direct correlation between the vigour of the proliferative response measured 3 days following exposure and the frequency of DC in draining nodes at 24 h. Collectively these data suggest that following skin sensitization the migration of DC into the draining lymph nodes influences quantitatively the primary immune response and the development of contact allergy.