Immunohistological analysis of 'negative' patch test sites in atopic dermatitis

Variable results have been obtained when patients with atopic dermatitis (AD),) have been patch tested with allergens known to produce a positive prick test. The significance of patch test results in our understanding of the pathogenesis of AD therefore remains questionable.

This study was designed to determine the relevance of either positive or negative patch test results in relation to the expression of cell mediated immunity to allergens in patients with AD.

Thirty-five patients with AD exhibiting patch test positivity to one or more aeroallergens on 'tape stripped' areas of the back were retested without prior tape stripping. Nine patients again showed positivity to one or more allergens while 26 failed to show positive reactions. In six of the positive patients both positive and negative patch tests were observed. Skin biopsies were taken from these matched positive and negative patch test sites as well as from an area of uninvolved skin. Samples were frozen and cryostat sections were analysed with immunohistological techniques using monoclonal antibodies to investigate the distribution of immunocompetent cells.

All positive patch tests exhibited characteristics of a cell mediated immune response. The negative patch test sites were also found to contain evidence of mononuclear cell infiltration. Both negative and positive patch test sites showed significantly greater proportions of T cells compared to uninvolved skin. No increase in numbers of RFD1 positive and RFD7 positive macrophages were observed in either positive or negative patch test sites. Expression of CD23 by CD1 positive Langerhans cells was raised in both negative and positive patch tests compared to uninvolved areas. A significant increase in the population (RFD7+, CD23+) was seen in positive patch test sites compared to uninvolved skin. An increase in the proportion of RFD1 positive cells expressing CD23 was also seen in both negative and positive patch tests compared to uninvolved skin.

This paper demonstrates that immunological reactions are promoted at 'non-tape stripped' patch test sites where no clinical evidence of reactivity is seen. Together the data demonstrate that the presence of systemic cell mediated immunity to specific allergens identified in patients by positive patch test, may also be present when no clinical signs are seen at the patch test site.