Human lymphocytes can generate thymus-independent as well as thymus-dependent anti-hapten plaque-forming cell responses in vitro.

Human lymphocytes from peripheral blood, tonsils, and adenoids produced hapten-specific anti-TNP antibodies after in vitro stimulation with TNP-Ba. This antigen did not appear to behave as a polyclonal activator since TNP-Ba-stimulated cells lysed TNP-conjugated sheep erythrocytes (SRBC) but not unconjugated SRBC or DPPC-conjugated SRBC, whereas cells from PWM-stimulated cultures formed hemolytic plaques with both haptenated and unhaptenated targets. In addition, TNP-Ba generated PFC were inhibitable by soluble hapten (TNP-EACA or TNP-BSA). B cells depleted of T cells (less than 1% E-RFC cells) were able to respond to TNP-Ba but not to PWM or TNP-KLH. When varying ratios of T and B cells were added to cultures the TNP-Ba anti-TNP response was found to have a linear correlation with the number of B cells added (r = 0.987) and was maximal in the absence of added T cells. This was in contrast to the response to TNP-KLH, which required T cells at a particular T:B ratio for optimal induction of anti-TNP PFC. Thus, both T-dependent and T-independent anti-hapten responses could be elicited in human lymphoid cell cultures.