Background Information

Delayed-type hypersensitivity (DTH) is an in vivo T cell-dependent immune response manifested as an inflammatory reaction that reaches peak intensity 24 to 48 hr after antigenic challenge. The DTH reaction occurs at the site of antigen deposition; the skin serves as the usual site for eliciting DTH in experimental systems. Typical sites of challenge are the footpad or the pinna. Histologically, DTH reactions are characterized by an intense mononuclear induration, with limited polymorphonuclear cell involvement (Sell, 1987). DTH reactions can also occur within an organ (e.g., the brain) initiating a pathology characteristic of the tissue-specific autoimmune diseases such as multiple sclerosis or experimental autoimmune encephalomyelitis (Zamvil and Steinman, 1990).

DTH reactions

Two forms of DTH reactions—tuberculin-type DTH and contact sensitivity—have been described. These two are primarily distinguished by the manner in which the response is elicited. Classical tuberculin-type DTH responses in humans are elicited by subcutaneous injection of antigen. In contact sensitivity, the sensitization and challenge phases generally involve epicutaneous application of a chemically reactive hapten. The hapten is thought to modify autologous molecules in the skin resulting in priming or recruitment and activation of antigen-specific T cells to the involved site. Additional features of contact sensitivity are discussed in unit 4.2.

The sensitization and challenge phases of the DTH reaction represent a continuum of the immune response. The initial sensitization period is generally six days, after which the mice are challenged with antigen at a distal site. The sensitization period is accompanied by the homing of antigen-specific cells, which mediate DTH responses, to virtually all of the periperal lymphoid tissues. Once activated, these cells are poised to mount a DTH response at any local site challenged with antigen.

Assays of DTH response

Although numerous in vitro tests have been developed to assess T cell activity, these in vitro assays do not necessarily monitor the same cells which mediate DTH nor do they consider the influences of regulatory events which may influence in vivo T cell function. Therefore, local DTH responses represent an important source of information concerning in vivo T cell function. Clinically, DTH skin tests are of value in the overall assessment of immunocompetence. The inability of human adults to react to a battery of common skin antigens suggests hy-poreactivity or a defect in cellular immunity.

Measuring the intensity of DTH involves quantitating some aspect of the local inflammatory response. In humans and guinea pigs this is readily done by measuring the redness and induration of an area of shaven skin. In mice, this approach has failed. However, there are two assays of measuring DTH—measurement of swelling and determination of cellular infiltra-tion—that have proven reliable.

Swelling responses. Swelling assays in which thickness gauge calipers are used to measure swelling responses are the most

In Vivo Assays for Lymphocyte Function

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