BACKGROUND The B subunit of
Escherichia coli heat-labile enterotoxin (LTB) is a potent mucosal immune adjuvant. However,
there is little information about LTB’s potential as a parenteral adjuvant.
OBJECTIVES We aimed at evaluating and better understanding rLTB’s potential as a parenteral adjuvant using the fused R1
repeat of
Mycoplasma hyopneumoniae
P97 adhesin as an antigen to characterise the humoral immune response induced by this
construct and comparing it to that generated when aluminium hydroxide is used as adjuvant instead.
METHODS BALB/c mice were immunised intraperitoneally with either rLTBR1 or recombinant R1 adsorbed onto aluminium
hydroxide. The levels of systemic anti-rR1 antibodies (total Ig, IgG1, IgG2a, and IgA) were assessed by enzyme-linked immunosorbent
assay (ELISA). The ratio of IgG1 and IgG2a was used to characterise a Th1, Th2, or mixed Th1/Th2 immune response.
FINDINGS Western blot confirmed rR1, either alone or fused to LTB, remained antigenic; anti-cholera toxin ELISA confirmed that
LTB retained its activity when expressed in a heterologous system. Mice immunised with the rLTBR1 fusion protein produced
approximately twice as much anti-rR1 immunoglobulins as mice vaccinated with rR1 adsorbed onto aluminium hydroxide.
Animals vaccinated with either rLTBR1 or rR1 adsorbed onto aluminium hydroxide presented a mixed Th1/Th2 immune
response. We speculate this might be a result of rR1 immune modulation rather than adjuvant modulation. Mice immunised with
rLTBR1 produced approximately 1.5-fold more serum IgA than animals immunised with rR1 and aluminium hydroxide.
MAIN CONCLUSIONS The results suggest that rLTB is a more powerful parenteral adjuvant than aluminium hydroxide when
administered intraperitoneally as it induced higher antibody titres. Therefore, we recommend that rLTB be considered an
alternative adjuvant, even if different administration routes are employed.
