Ia antigen-bearing (Ia+) Langerhans cells have attained an important position as immunocompetent cells in the epidermis. Recently there have been successive reports on other new possible candidates for immunocompetent cells in the epidermis, i.e., Ia+ keratinocytes and dendritic Thy-1 antigen-bearing (Thy-1+) epidermal cells which also express Ly-5 antigen and asialo-GM1. Based on our previous findings that in allergic contact sensitivity reactions, keratinocytes express la antigen 3-9 days postchallenge, in this report, we have attempted to define more clearly the dynamic changes of Ia+ keratinocytes and dendritic Thy-1+ epidermal cells by enumeration of the precise percentages of Ia+, Thy-1+, and Ly-5 antigen-bearing (Ly-5+) cells in epidermal cells at various times of the challenge phase in allergic contact sensitivity reactions by use of a fluorescence-activated cell sorter. By 24 h postchallenge, the percentages of Ia+, Thy-1+, and Ly-5+ cells showed hardly any change. There were approximately 2% Ia+ cells, 50% Thy-1+ cells which consist of 2 populations (i.e., 45% weakly Thy-1 antigen-positive cells and 4% strongly Thy-1 antigen-positive cells), and 3.5% Ly-5+ cells. From 48 h postchallenge, however, the percentage of Ia+ cells and that of Thy-1+ cells began to increase and reached a plateau, with approximately 20% Ia+ cells and 70% Thy-1+ cells, respectively, at 120 h postchallenge. The change of the percentages of Ly-5+ cells appears to correspond to that of strongly Thy-1 antigen-positive cells. Only at 48 h postchallenge, Ly-5+ cells and strongly Thy-1 antigen-positive cells showed a small increase in number, comprising approximately 10% of the epidermal cells. These data suggest that among Thy-1+ epidermal cells, strongly Thy-1 antigen-positive cells correspond to dendritic Thy-1+ epidermal cells, and in contact sensitivity reactions in mice, dendritic Thy-1+ epidermal cells show only a minor dynamic change in contrast to Ia+ cells, in which more than 15% of keratinocytes express Ia antigen from 48 h postchallenge.
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