Actinobacillus pleuropneumoniae, a porcine respiratory tract pathogen, has been shown to express transferrinbinding proteins and urease during infection. Both activities have been associated with virulence; however, their functional role for infection has not yet been elucidated. We used two isogenic A. pleuropneumoniae single mutants (⌬exbB and ⌬ureC) and a newly constructed A. pleuropneumoniae double (⌬ureC ⌬exbB) mutant in aerosol infection experiments. Neither the A. pleuropneumoniae ⌬exbB mutant nor the double ⌬ureC ⌬exbB mutant was able to colonize sufficiently long to initiate a detectable humoral immune response. These results imply that the ability to utilize transferrin-bound iron is required for multiplication and persistence of A. pleuropneumoniae in the porcine respiratory tract. The A. pleuropneumoniae ⌬ureC mutant and the parent strain both caused infections that were indistinguishable from one another in the acute phase of disease; however, 3 weeks postinfection the A. pleuropneumoniae ⌬ureC mutant, in contrast to the parent strain, could not be isolated from healthy lung tissue. In addition, the local immune response-as assessed by fluorescenceactivated cell sorter and enzyme-linked immunosorbent spot analyses-revealed a significantly higher number of A. pleuropneumoniae-specific B cells in the bronchoalveolar lavage fluid (BALF) of pigs infected with the A. pleuropneumoniae ⌬ureC mutant than in the BALF of those infected with the parent strain. These results imply that A. pleuropneumoniae urease activity may cause sufficient impairment of the local immune response to slightly improve the persistence of the urease-positive A. pleuropneumoniae parent strain.Actinobacillus pleuropneumoniae is the etiologic agent of porcine pleuropneumonia, a highly infectious disease of fattening pigs occurring worldwide (12). A number of putative virulence factors, such as Apx toxins, capsule, lipopolysaccharide (LPS), the ability to utilize transferrin-bound iron, and urease, have been described elsewhere (18). To date, conclusive evidence obtained by challenge experiments has been presented to confirm the role of Apx toxins and capsular material. A spontaneous Apx toxin-negative A. pleuropneumoniae strain was shown to be avirulent (14), and this result was supported later by using transposon mutagenesis (36) as well as by an isogenic A. pleuropneumoniae apxC insertion mutant (29). Also, capsule-deficient A. pleuropneumoniae strains obtained by chemical mutagenesis were shown to be attenuated (22), and this result was confirmed by reconstituting virulence properties and capsule formation upon transformation with a recombinant plasmid (39). Also, it was shown recently that the [Cu,Zn]-superoxide dismutase is not required for virulence (35). For other putative virulence factors, such as LPS (1, 3, 4), and the utilization of transferrin-bound iron (15,17,40), no conclusive challenge experiments have been performed to date. With respect to urease, data are inconclusive; ureasenegative A. pleuropneumoniae mutants hav...

The gut immune system is an essential part of the bamer function of the gut wall. The uptake of microbial and nutritional antigens in the gut starts immune responses by initiating either tolerance or a specific secretory immune reaction (for review see refs. 1-4). Large numbers of lymphocytes are distributed in the epithelium and LP of the gut mucosa. Some of the LP lymphocytes are PP-derived B lymphoblasts, becoming mature plasma cells in the LP. They preferentially produce IgA (5, 6). The majority of T cells in the LP are CD4+ (7,8). The T cells show a high expression of genes associated with cell activation (9). The proportion of antigen-primed memory cells is higher in LP T cells than in peripheral blood lymphocytes (reviewed in 4).In the early postnatal period, the gut wall is exposed to microbial antigens for the first time. Additionally, the amount of

As a result of the European ban of in-feed growth-promoting antibiotics, new strategies are being developed to increase the resistance to disease in farm animals. In pig production, this is of particular importance during the weaning transition when piglets are subjected to major stressful events, making them highly sensitive to digestive disorders. At this time, the development of both innate and adaptive immunity at the mucosal surface is critical in preventing the potential harmful effects of intestinal pathogenic agents. Strategies aiming at stimulating natural host defences through the use of substances able to modulate immune functions have gained increasing interest in animal research, and different bioactive components a priori sharing those properties have been the subject of in vivo nutritional investigations in pig. Among these, yeast derivates (b-glucans and mannans) are able to interact with immune cells, particularly phagocytic cells. However, studies where they have been fed to pigs have shown inconsistent results, suggesting that their ability to target the sensitive immune cells through the oral route is questionable. The plant extracts, which would benefit from a positive image in the public opinion, have also been tested. However, due to a lack of data on the bioactive components of particular plants and the large diversity of species, it has proved difficult to prepare extracts of equivalent potency and thus, the literature on their influence on pig immunity remains inconclusive. In considering piglet immunity and health benefits, the most promising results to date have been obtained with spray-dried animal plasma, whose positive effects would be provided by specific antibodies and nonspecific competition of some plasma components with bacteria for intestinal receptors. The major positive effect of spray-dried animal plasma is in reducing the infiltration of gut-associated lymphoid tissue by immune cells, which is likely to be the result of a decreased colonisation by potentially harmful bacteria. This review also highlights the limitations of some of the published in vivo studies on the immunomodulatory activity of certain feed additives. Among those, the lack of standardisation of extracts and the heterogeneity of piglet-rearing conditions (e.g. exposure to pathogens) are likely the most limiting.

Dr Mick Bailey, fax +44 117 928 9505, email Mick. Bailey@bris.ac.uk In human disease and rodent models, immune responses in the intestinal mucosa can be damaging. Damage is characterised by villus atrophy, crypt hyperplasia and reduced ability to digest and absorb nutrients. In normal individuals active responses to harmless environmental antigens associated with food and commensal bacteria are controlled by the development of immunological tolerance. Similar pathological changes occur in piglets weaned early from their mothers. Active immune responses to food antigens are observed in these piglets, and we and others have hypothesised that the changes occur as a result of transient allergic immune responses to novel food or bacteria antigens. The normal mechanism for producing tolerance to food antigens may operate at induction (Peyer's patches and mesenteric lymph nodes) or at the effector stage (intestinal lamina propria). In our piglet studies immunological tolerance occurs despite the initial active response. Together with evidence from rodents, this observation suggests that active responses are likely to be controlled at the effector stage, within the intestinal lamina propria. Support for this mechanism comes from the observation that human and pig intestinal T-cells are susceptible to apoptosis, and that this process is accelerated by antigen. We suggest that the role of the normal mature intestinal lamina propria is a balance between immunological effector and regulatory function. In neonatal animals this balance develops slowly and is dependant on contact with antigen. Immunological insults such as weaning may tip the balance of the developing mucosal immune system into excessive effector or regulatory function resulting in transient or chronic allergy or disease susceptibility.