Immunization Against Active Ghrelin Using Virus-Like Particles for Obesity Treatment

Abstract

Ghrelin is a gut hormone that stimulates food intake. In physiological conditions, ghrelin plasma levels rise with fasting and decrease after meals. Obese individuals have low fasting ghrelin levels that rise after food restriction, which is pointed out as a reason for the difficulty in maintaining weight loss. Some bariatric surgery procedures prevent rise in ghrelin levels with weight loss and this has been hypothesised to contribute to the long-term success of the treatment. The main goal of this study was to develop a safe and effective anti-ghrelin vaccine for obesity, through the chemical conjugation of ghrelin with a virus like particle, namely NS1 protein tubules from the Bluetongue Virus (BTV) using a hetero-bifunctional cross linker. Male adult C57BL/6 mice, with a normal weight and with diet-induced obesity (DIO), were randomized into six weight matched groups (n=6/group) and each group of mice received three intra-peritoneal injections with two weeks intervals, containing either 75 µg of ghrelin-NS1 immunoconjugate, 75 µg of NS1 or PBS. Our data show that immunized animals present increasing titres of anti-ghrelin antibodies, while their cumulative food intake significantly decreased and energy expenditure was significantly enhanced, although there were no significative changes in body weight.Vaccinated DIO mice also displayed significant decrease of NPY gene expression in the basal hypothalamus reflecting a decrease in central orexigenic signals. This study suggests that this anti-ghrelin vaccine has a positive impact on energy homeostasis and may be an additional therapeutical tool to be used with diet and exercise for obesity treatment.

Obesity vaccines

“…The ability of the vaccine to trigger an immune response was tested in normal weight and DIO male mice that developed increasing titers of specific anti-ghrelin antibodies, confirming that a vaccine consisting of an immunoconjugate alone was able to trigger an immune response without the need adjuvants. 80 The antibodies titers attained after the immunization protocol were not very high, when compared with antibodies titers after common infectious diseases, which is also reassuring in safety concerns, since complete ghrelin neutralization as a strategy for obesity treatment is not desirable as ghrelin also intervenes in several key biological processes besides appetite regulation, such as growth hormone secretion, gastro-intestinal and neurological functions. 56 In accordance to the expected biological effects of ghrelin neutralization, vaccinated mice showed increased energy expenditure that usually translates into greater ease of weight loss and maintenance, as ghrelin is known to suppress energy metabolism by increasing the respiratory quotient through decreasing utilization of fat as energy.…”

“…The main goal of this VLP based approach was to develop a safer and more effective anti-ghrelin vaccine that could be used for human treatment. 80 NS1 protein tubules, although not part of the viral capsid, possess the same immunogenic characteristics as classical VLPs 81,82 . The choice of NS1 tubules as VLP-like carrier protein was driven by its previous use as a distribution system for molecules of prophylactic vaccines against common human infectious diseases, such as foot-and-mouth disease and influenza A virus 82,83 .…”

“…84 In comparison, in vaccinated normal weight mice there were no significant differences in the genetic expression of NPY gene but the expression of POMC was significantly lower in the basal hypothalamus, the fact that these findings where only observed in normal weight mice, suggests the likelihood of a compensatory mechanism to the decreased orexigenic signals from the periphery in order to prevent a reduction in the feeding threshold of normal weight mice. 80 Anti-ghrelin vaccination was associated with a paradoxical increase in circulating ghrelin levels that did not appear to have biological effects in energy regulation and were proven to be due to the presence of circulating immune complexes of ghrelin-antighrelin. 80 The rise in ghrelin levels after other vaccination strategies had been previously reported although the presence of circulating immunocomplexes was not documented.…”

“…80 Anti-ghrelin vaccination was associated with a paradoxical increase in circulating ghrelin levels that did not appear to have biological effects in energy regulation and were proven to be due to the presence of circulating immune complexes of ghrelin-antighrelin. 80 The rise in ghrelin levels after other vaccination strategies had been previously reported although the presence of circulating immunocomplexes was not documented. 75 The presence of circulating immunocomplexes that could be due to a lower elimination rate, has not been shown to suffer deposition in the kidney glomerular basement membrane or induce renal toxicity.…”

“…75 The presence of circulating immunocomplexes that could be due to a lower elimination rate, has not been shown to suffer deposition in the kidney glomerular basement membrane or induce renal toxicity. As the anti-ghrelin vaccination did not interfere with ghrelin expression in the stomach, ghrelin appears to be normally synthetized in immunized animals and after neutralization of ghrelin biological activity does not seems to occur upregulation of ghrelin expression in order to maintain the homeostasis 80 ( Table 1). Since ghrelin is a growth hormone secretagogue, ghrelin neutralization could induce alterations in GH/IGF-1 axis, 85 yet anti-ghrelin vaccination had no effect in GH and IGF-1 levels, suggesting the unlikelihood to cause endocrine adverse effects on the growth hormone axis.…”