3.4 How could a vaccine work?

Vaccination outcome is affected by many different factors. In general these factors are pathogen-related such as antigenic variation, avail­ability of in vitro culture system and latency and persistence plus host-related factors like genetic variation and tissue variability (mucosa, brain).

Thus vaccine requirements are:
(1) activation of antigen-presenting cells to initiate antigen processing and production of interleukins,

(2) activation of both T cells and B cells to give a high yield of memory cells,

(3) generation of helper T and cytotoxic T cells to several epitopes; to overcome the variation in the immune response in the population due to MHC polymorphism and

(4) persistence of antigen, probably in dendritic cells in lymphoid tissue, where memory B cells are recruited, to form antibody-secreting cells that will continue to produce antibody.

Immunity is the evolutionary product of host defence against specific pathogens. The host responds to unique properties of pathogens that are fully expressed in an active infection. Many of these properties do not exist in killed vaccines. Specific parts of the body react in specific ways to an infection therefore; the natural route of infection is the most obvious route for vaccination.

Enterisol^® Ileitis

The advantages of an attenuated live vaccine such as Enterisol^® Ileitis which mimicks the natural oral route of infection are high efficacy and long duration of immunity. It stimulates innate danger signals by replication and amplification of antigen mass. And as all antigens are presented it induces the best humoral and cellular response.

Development of protective immunity to Lawsonia intracellularis has been demonstrated in pigs during controlled challenge trials and observed in sequential natural disease outbreaks. These studies suggest that a modified live vaccine may induce protective immunity to PE. Enterisol^® Ileitis is a modified live oral vaccine that has the ability to induce both humoral and cellular immune responses. Evaluation of the vaccine in standardized challenge models has shown that it effectively protects animals that are challenged with either a homologous or heterologous Lawsonia intracellularis isolate (Kroll et al. 2004). These studies have shown a reduction of bacterial shedding, reduced lesion develop­ment and reduction of productivity losses in challenged pigs.