Differential effects of concanavalin A on T helper dependent and independent antibody responses

The in vivo immunosuppressive effects of Concanavalin A (Con A) on the thymus (T) helper dependent response to sheep erythrocytes (SRBC) and the T helper independent response to E. coli lipopolysaccharide 055: B5 have been investigated. Maximum suppression was observed in BALB/c mice treated with 3 successive ip injections of 100 μg each of Con A administered on Days ?1, 0, and +1 relative to the day of immunization (Day 0) with SRBC (splenic PFC on Day 4 reduced from 74,000 down to 1400). As little as 10 μg × 3 of Con A was capable of depressing both the PFC and serologic response while 2.5 μg × 3 was ineffective. A single ip injection of 300 μg of Con A administered simultaneously at the time of immunization with SRBC reduced splenic PFC from 74,000 down to 9990 and serum antibody titers by 3–4 log₂ units. Significant depression was noted if mice were treated 1, 2, or 3 days prior to but not following immunization. Immunosuppression was noted in mice which had been treated and immunized ip or iv or treated iv and immunized ip. Heat inactivation reduced if not abolished the immunosuppressive properties of Con A.Mice immunized with varying doses of a bacterial vaccine of E. coli 055: B5 (15–1500 × 10⁶ killed organisms) and treated with Con A on days ?1, 0, and +1 had no significant depression of splenic PFC when compared to nontreated controls. Mice treated with Con A and simultaneously immunized with both SRBC and E. coli had a 37-fold reduction in the PFC response to SRBC but only a 2-fold reduction in the response to E. coli. This differential immunosuppressive effect on T helper dependent and independent responses is consistent with the recently reported in vitro specificity which Con A has for theta antigen bearing lymphocytes.