Detection of contact dermatitis in rat skin by intravital microscopy

The sensitivity of fluorescence intravital microscopy for the detection of immunologically elicited inflammatory reactions in the skin was compared to assessment by tissue swelling measurement. Rats were immunized to the contact-sensitizing agent toluene diisocyanate (TDI) by its application to the skin and challenged by the same procedure at a different site one week thereafter. At 2, 6, or 12 hrs after challenge individual rats received a bolus of fluorescently labeled albumin intravenously. Vascular integrity was determined by estimating the surface of areas with increased vascular permeability as observed by fluorescence intravital microscopy; contact dermatitis (CD) was assessed just prior to infusion of labeled albumin determination of tissue swelling by a semi-automatic micrometer. At 12 hrs after challenge, CD was detectable by tissue swelling. Intravital fluorescence microscopy revealed massive extravasation of the fluorescent dye at the site of challenge. At 6 hrs after challenge, CD was not yet discernible by toe thickness measurements. However, by fluorescence intravital microscopy significant leakage of labeled albumin was observed. Thus, at 6 hrs after challenge fluorescence intravital microscopy was more sensitive than determination of tissue thickness increment for detection of a contact hypersensitivity reaction. At 2 hrs after challenge, CD was neither demonstrable by tissue swelling measurements, nor by fluorescence intravital microscopy. Therefore, an early component of a delayed-type hypersensitivity (DTH) reaction to a contact allergen as shown to occur in mice, remains to be demonstrated in the rat. On the other hand, fluorescence intravital microscopy revealed non-immunological skin irritation by a 5% TDI solution within 2 hrs after its application to the skin of non-immunized rats.