Anti-DNP antibody formation resulting from intraperitoneal (i.p.) immunization with DNP-KLH may be blocked by simultaneous (i.p.) injection of DNP-Ova or native Ova in mice orally tolerant to Ova, but not in normal mice. In Ova-tolerant mice the inhibition of anti-DNP antibody formation also occurred when DNP-Ova and DNP-KLH were given by separate routes of immunization: subcutaneous (s.c.) and i.p. A second exposure to Ova by gastric intubation (gavage) or intravenous administration simultaneously with i.p. immunization with DNP-KLH failed to inhibit anti-DNP antibody formation. There was inhibition of responses to DNP-KLH i.p. by DNP-Ova given 24 h before, but not 24 h after, and in the Ova-tolerant mice, addition of DNP-Ova only to the primary immunization with DNP-KLH inhibited secondary and tertiary responses to DNP-KLH in the absence of further exposures to DNP-Ova. These results suggest that the indirect effects of parenteral exposure of tolerant mice to the tolerated immunogen may inhibit unrelated immune responses. This inhibition is not due to 'innocent bystanding' suppression, i.e., to inhibitory cytokines provided locally by specific suppressor lymphocytes; it may derive from more durable perturbations of immune system.

Carvalho CR, Verdolin BA, Vaz NM. Indirect Effects of Oral Tolerance Cannot be Ascribed to Bystander Suppression. Scand J Immunol 1997;45:276-281 The addition of tolerated antigens to immunizing doses of unrelated antigens blocks antibody responses to these unrelated antigens. This inhibition, which the authors have called the indirect effects of tolerated antigens, occurs even when the mixture of proteins is injected as soon as 24 h after the oral tolerance induction. The indirect effects also do not require the simultaneous injection of the two proteins: they are still present 72 h after an injection of Ova in Ova tolerant mice, but do not occur if the unrelated protein is injected 24 h before the tolerated protein. In addition, indirect effects do not block secondary responses to unrelated proteins if the primary immunization is made in the absence of the tolerated protein. These results cannot be explained by innocent bystander suppression, which is believed to result from the action of suppressive cytokines released by specific tolerant lymphocytes upon unrelated lymphocytes that would otherwise respond to the second, non-tolerated antigen. Indirect effects may be better understood in terms of network models.

Initial contacts with a T-dependent antigen by mucosal routes may result in oral tolerance, defined as the inhibition of specific antibody formation after subsequent parenteral immunizations with the same antigen. We describe here an additional and permanent consequence of these initial contacts, namely, the blockade of secondary-type responsiveness to subsequent parenteral contacts with the antigen. When repeatedly boosted ip with small doses (3 µg) of ovalbumin (OVA) (or lysozyme), primed B6D2F1 mice showed progressively higher antibody responses. In contrast, mice primed after a single oral exposure to the antigen, although repeatedly boosted, maintained their secondary antibody titers on a level which was inversely proportional to the dose of antigen in the oral pretreatment. This phenomenon also occurred in situations in which oral tolerance was not induced. For example, senile 70-week-old B6D2F1 mice pretreated with a single gavage of 20 mg OVA did not become tolerant, i.e., they formed the same secondary levels of anti-OVA antibodies as non-pretreated mice. However, after 4 weekly challenges with 3 µg OVA ip, orally pretreated mice maintained the same anti-OVA serum levels, whereas the levels of control mice increased sequentially. This stabilizing effect of mucosal exposure was dose dependent, occurred with different proteins and was triggered by single or multiple oral or nasal exposures to the antigen.

Vaz NM, Faria AMC, Verdolin BA, Carvalho CR. Immaturity, Ageing and Oral Tolerance. Scand J Immunol 1997;46:225-229 Founding studies of cellular immunology emphasized that tolerance to allografts could only be achieved early in the embryonic or neonatal period, suggesting that the establishment of self-tolerance, a main event in the organization of the immune system, would necessarily take place in immature hosts. Contradicting these ideas, oral tolerance is a common, daily phenomenon, easily achieved by a physiological route in adult immunocompetent animals. Furthermore, there is solid evidence that, after the neonatal period, the susceptibility to oral tolerance induction also wanes and that it may be restored by adoptive transfer of cells from young hosts. These findings are briefly reviewed here to emphasize that immunological activity is a continuous and ongoing epigenesis extending throughout the entire life of the organism, far beyond the early phases of ontogenesis. Nelson M. Vaz, Department of Biochemistry and Immunology, CP 486, Instituto de Ciências Biológicas, Universidade Federal de Minas Gevais 30161-970, Belo Horizonte, MG, Brazil THE DOMINANT IMMUNOLOGICAL PARADIGMWe refer to the 'dominant immunological paradigm' [1] as a large interlaced set of terms and notions based on the concept of specific immunological recognition of foreign epitopes (immunity) and its complement, the notion of self-tolerance (self/nonself discrimination). The current version of the paradigm is based in different versions of the Clonal Selection Theory of Antibody Formation [2]. An alternative paradigm should be able to define, discuss and explain immunological activities and immunological phenomena without relying on the distinction between foreign and familiar materials (self/non-self discrimination) or on the notion of self-tolerance (self-ignorance). Up to this moment, we are not aware of such an alternative being fully formulated.In a previous essay [3], we pointed to a confusion of domains of description inherent to the current paradigm, which confounds the structural dynamics of the immune system (its physiology) with immunological phenomena (such as specific immune responses) which take place in the domain of interactions of the organism acting as a whole 1 . In the present essay, we use the same way of seeing to describe the set of phenomena currently described as 'oral tolerance'. We will show that these phenomena are not explained by the current paradigm and require alternative systemic notions to explain them. SELF-TOLERANCE AND IMMATURITYCreated in the early 1960s, the current immunological paradigm was heavily influenced by experimental findings on bovine twins' natural allogeneic chimeras [4], the rejection of allografts in mice [5,6] and chickens [7,8] and the adoptive transfer of immunological competence by lymphocytes [9][10][11][12]. The Clonal Selection Theory hinged around the notions of self/non-self discrimination (self-tolerance) already emphasized in a previous book by Burnet & Fenner [13], exp...

In the present review we address oral tolerance as an important biological phenomenon and discuss how it is affected by aging. Other factors such as frequency of feeding and previous digestion of the antigen also seem to influence the establishment of oral tolerance. We also analyze immunoglobulin isotypes of specific antibodies formed by tolerant and immunized animals of different ages submitted to different conditions of oral antigen administration. Isotypic patterns were studied as a parameter for assessing the pathways of B and T cell interactions leading to antibody production.