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In contrast to systemic autoimmunity, spontaneous autoimmune thyroiditis of Obese strain (OS) chickens is associated with a marked T cell hyperreactivity in vitro , i.e. an increased proliferation and interleukin 2 (IL 2) secretion in response to Concanavalin A (ConA). In the present study we report an enhanced capacity of OS peripheral lymphoid cells (splenocytes and peripheral blood lymphocytes, PBL) to adsorb fluorescein isothiocyante (FITC) labelled ConA, but not phytohemagglutinin (PHA). However, the elevated ConA binding cannot be a prerequisite for in vitro ConA hyperreactivity as OS thymocytes are normal with respect to ConA binding but nonetheless exhibit elevated responses to this mitogen. Moreover, ConA binding does not correlate with the frequency of cells able to express IL 2 receptors upon short term ConA stimulation. The percentage of ConA activatable cells was found to be increased in OS- PBL as compared to normal control PBL, but was unaltered in OS splenocytes. This finding points to a further mechanism of T cell hyperreactivity in OS chicks in addition to the previously reported defects in nonspecific immunosuppression. Finally, enumeration of cells in the S phase revealed that enhanced proliferation of OS T lymphocytes was not restricted to the in vitro response to ConA and phytohemagglutinin (PHA) but also occurs in vivo .

Monoclonal antibody INN-CH 16 recognizes a surface determinant exclusively present on activated chicken T lymphocytes. The present experiments suggest that this chicken activated T lymphocyte antigen (CATLA) represents the chicken IL-2 receptor or an associated structure, as (i) the kinetics of CATLA expression during T cell activation are analogous to those described for the mammalian receptor for IL-2, (ii) the IL-2 dependent proliferation of mitogen prestimulated chicken lymphocytes is competitively inhibited by INN-CH 16, and (iii) pretreatment of T lymphoblasts with INN-CH 16 drastically reduces their capacity to absorb IL-2 activity from supernatants of mitogen activated chicken lymphocytes.