While afi T cells in mammals may express one of many variable (V) gene families in the 13 had greater than normal numbers of Vp2' T cells and appeared as healthy as thymectomized and untreated controls. While production of IgM and IgG antibodies was unimpaired, IgA antibody production was severely compromised in the Vpldepleted birds. The levels of secretory IgA in bile and lung lavage fluid were reduced 1000-to 10,000-fold and secretory IgA antibodies were not produced in response to mucosal immunization. B-cell production of IgA antibodies thus appears to require T cells expressing the Vp1 genes, whereas T cells that express the Vp2 genes lack this capacity.Most of the T cells in mammals express an antigen receptor composed of a and , f chains, while the others express a T-cell receptor (TCR) composed of 'y and 8 subunits (1). Avian homologues of the mammalian a/3 and y5 TCRs can be identified with monoclonal antibodies. The chicken ySTCR is identified with the TCR1 antibody (2) and two types of af3TCR are identified in gallinaceous birds with the TCR2 and TCR3 antibodies (3)(4)(5). The key to this unexpected finding is provided by the recent cloning of genes in the chicken TCR X3 locus. This P locus contains two variable (V) gene families, one diversity (D) gene, four joining (J) genes, and a single constant region (C) gene (6). Chicken a3 T cells therefore may express either V91 or V,2 genes, the products of which can be recognized, respectively, by the TCR2 and TCR3 antibodies (7). While there is little sequence homology (30%) between the Vp1 and Vp2 genes, members within the two families are very similar (>95% homology). Thus, to an even greater extent than in mammals, which have many Va gene families (1), TCR(3 diversity in birds is generated primarily by junctional variations created by V-D-Jp splicing and insertion of nonencoded nucleotides (6). Interestingly, the numerous Va genes in mammals can be assigned to two superfamilies on the basis of invariant sequences (8): V91 superfamily gene members encode an arginine at position 64 and aspartic acid at position 86 to form a salt bridge between these residues, while V,2 genes instead feature an invariant codon for tyrosine at position 65. These features are retained in the chicken V91 and Vp2 genes, respectively (6).The chicken therefore provides a relatively simple model for study of the functional roles of acB T cells by using the two prototype Va genes. In earlier studies, we found that development ofthe TCR2 thymocyte population can be selectively inhibited by embryonic administration of the TCR2 (V,81 specific) monoclonal antibody (9, 10). When this antibody treatment is combined with surgical removal of the thymus after hatching, sustained suppression of the TCR2 cell population can be achieved in young birds. In the present study, we confirmed the selective suppression of TCR2' cells and observed a profound effect of this treatment on the integrity of mucosal immunity. MATERIALS AND METHODSChickens. Chickens hatched from fertile outbred Wh...