Anti-CD40L treatment does not prevent oral tolerance as measured by suppression of systemic priming of CTLs. CTL activity in response to intravenous priming with OVA-coated splenocytes (a) or to subcutaneous priming with OVA in CFA (b) is similarly suppressed after oral OVA given to mice treated with control mAb 6C8 (squares) and those given anti-CD40L mAb MR1 (circles). Mice were injected with 6C8 or MR1 (250 μg intraperitoneally), then fed either PBS (open symbols) or OVA in PBS (filled symbols). After 14 days, mice were primed systemically. Seven days later they were killed and their splenocytes were recovered for a standard in vitro ⁵¹Cr release assay. CTL activity was expressed as percent OVA-specific lysis; the percent of total radioactivity, corrected for background, released from ⁵¹Cr-loaded, OVA_257-264 peptide-coated target cells (T) by primed effector spleen cells (E). CTL activity plots for individual mice (shown in a and b) were converted into lytic units per spleen from four experiments (c) in which mice received either PBS or 20 mg oral OVA in PBS on three alternating days and were primed as in a, and from two experiments (d) in which mice received either PBS or 0.5 mg oral OVA in PBS on five alternating days and were primed as in b. Horizontal bars are mean values.