The prickly lettuce agglutinin. II. Immunobiological properties

The prickly lettuce agglutinin (PLA), extracted from leaves of the plant Lactuca scariole, was tested for immunomodulatory effects on C57BL/6 mice. When administered with antigen in vivo, 50 micrograms PLA caused an enhancement of the primary splenic plaque-forming cell (PFC) response to the helper T cell-dependent antigens sheep red blood cells and trinitrophenyl-keyhole limpet hemocyanin; higher doses of PLA (200 micrograms) resulted in depressed responses to these antigens. Similarly, 50 micrograms PLA enhanced the B cell response to SIII, a helper T cell-independent antigen, but higher doses did not cause significant depression of the SIII-specific PFC response. When PLA (less than 25 micrograms) was coincubated with spleen cells and antigen, sheep red blood cells or SIII, in vitro the primary PFC response was enhanced; higher doses of PLA (greater than or equal to 25 micrograms) depressed cell culture viability and also depressed the resultant PFC responses. In the in vitro studies, stimulatory doses of PLA were effective only when added to cultures before or with antigen. In tests for mitogenic effects of PLA on spleen cells, PLA was found to be nonstimulatory. On the basis of these observations, we suggest that PLA may modulate murine splenic primary B cell responses by providing a stimulatory signal to B cells. The nature of the suppressive effect in vivo is unknown but may be attributable to antigenic competition.