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We have isolated 10 rat T-cell clones from the spleen or lymph nodes of seven different donors, These rats were immunized with 2-5 × 108 killed Actinobacillus actinomycetemcomitans (Aa) bacteria, injected either subcutaneously (s.c.) in complete Freund's adjuvant or intraperitoneally (i.p.) in saline. Clones studied to date have demonstrated a T-helper (Th) phenotype W3/13+ W3/25+ OX8- and OX22-. Clones were not stimulated in vitro by purified Aa-lipopolysaccharide (LPS) or heterologous Gram-negative bacteria, but proliferated when stimulated by bacteria representative of each of the three serological groups of Actinobacillus, indicating specificity for an Actinobacilluscommon antigen other than LPS. One clone (A4) proliferated vigorously when stimulated with concanavalin A (Con A) in vitro, produced interleukin-2 (IL-2) and was provisionally classified as a Th1 type. This appears to be one of the few Th !-type rat clones reported. All other clones tested did not produce IL-2, exhibited B-cell help to some extent, did not induce delayed-type hypersensitivity (DTH) when injected into the footpads of naive rats along with the specific antigen, and were classified as Th2 type. Adoptive transfer of 106 cells of one Th2-type Aa-specific clone into syngeneic recipients resulted in a specific splenocyte in vitro response to Aa 12-14 weeks after cell transfer, indicating survival of cloned cells in recipient animals. The use of such clones in studies of experimental periodontal disease is discussed. [ABSTRACT FROM AUTHOR]