the new age. cytokines as vaccine adjuvants, page-2

Due to the recent discovery of a number of chicken cytokine genes, studies have been performed to assess cytokine adjuvant activity. In comparison to mammalian cytokines, the adjuvant activities of chicken cytokines are less well characterized. Nevertheless, there are some chicken cytokines that have been examined for their potential use as vaccine adjuvants. One such cytokine is chicken IL-1 (ChIL-). The immunoadjuvant activity of ChIL-1 has been assessed using tetanus toxoid as an antigen. ChIL-1 increased antibody responses when administered in vivo as a recombinant protein compared to antigen alone45. Combined administration of ChIFN-, ChIFN- and ChIL-1 increased antibody responses to tetanus toxoid in an additive manner45. This supports the possibility that cytokines in synergy could be effective adjuvants and immune responses may be better when used in combination.

Similarly, the therapeutic and antiviral effect of ChIFN- has been demonstrated by Karaca and colleagues: coexpression of Newcastle disease virus (NDV) and ChIFN- in a Fowlpox vector (FPV) protected birds from challenge with NDV46. When delivered by FPV, NDV genes gave protection to birds after challenge with NDV; however, the chickens had lost weight due to the fowlpox virus. When ChIFN- was co-administered with the NDV gene in FPV, the birds did not lose bodyweight compared to non-vaccinated birds. This supports the concept that ChIFN- treatment prevents diseases through its antiviral activity whilst maintaining vaccination potential.

Most chicken cytokines are at the stage of in vitro characterization before assessment of activity as adjuvants in vivo. The adjuvant efficacy of IL-2 has not been demonstrated in chickens as yet. One study has been reported that shows that ChIL-2 is ineffective in enhancing the efficacy of a vaccine against Eimeria acervulina47. To assist our understanding of its role and evaluate its activity as a potential adjuvant, we have correlated the effects of ChIL-2 on immune cell populations in vitro and in vivo48. In vivo administration of ChIL-2 and measurement of the changes in T-cell populations showed that rChIL-2 induced the expression of cell surface IL-2 receptor (CD25) on peripheral blood lymphocytes as well as increased the proportion of CD4+ and CD8+ T cells (Fig. 1). Using bromodeoxyuridine (BrdU) incorporation as a measurement of cell proliferation, an increase in T-cell populations was found due to cell proliferation in response to ChIL-2 (Fig. 2). The ability of ChIL-2 to enhance T-cell activation and proliferation suggests it may have the potential to augment the immune response when used as an adjuvant.

In addition, we have previously shown the adjuvant ability of ChIFN- when chickens were injected with SRBC with or without ChIFN-49. Treatment with ChIFN- had no significant effect on the primary antibody response, but increased the secondary (IgG) antibody response (Fig. 3). Furthermore, ChIFN- allowed a 10-fold lower dose of antigen to be used. These studies support IFN- as having adjuvant activity and being used in vaccine formulations for better protection against disease.

Until recently, the adjuvant function of chicken myelomonocytic growth factor (cMGF) was not characterized. cMGF enhances the growth of macrophages and granulocytes from avian bone marrow progenitor cells50. The macrophage activating ability of cMGF is potentially important in controlling viral disease, and exploration of its role as an adjuvant is of particular interest. The in vivo biological activity of cMGF delivered via a live fowlpox virus (fp/cMGF) had been demonstrated in a previous study51. A more recent study has indicated the adjuvant potential of cMGF52. Birds highly susceptible to Marek's disease virus (MDV) were treated with fp/cMGF. The birds were then vaccinated with a commercial vaccine, herpes-virus of turkeys (HVT) 4 days later and challenged with a virulent (RB-1B) strain of MDV 1 week after cytokine treatment. In this trial, non-vaccinated birds suffered 100% mortality after challenge. Birds vaccinated with HVT alone suffered 33% mortality whereas vaccinated birds that were treated with cMGF had no mortality (Fig. 4). These results showed improved protection with the vaccine and less tumour incidence in MDV-challenged birds52, and clearly indicates the potential use of cMGF as a vaccine adjuvant for viral strains used in vaccines to increase protection against MDV