Cow's milk allergy (CMA), an immunologically mediated reaction to cow's milk proteins [
1], is one of the most prevalent human food-borne allergies, particularly in infants and young children. In North America, incidence of CMA is estimated at 2.5% in children and about 1% in adults with a 75% outgrowing rate at 16 years of age [
2]. Milk protein comprises a mixture of multiple proteins, including whey (such as β-lactoglobulin, α-lactalbumin and bovine serum albumin) and casein (such as α-S1-, α-S2-, β-, κ-, and γ-caseins) proteins. Hypersensitivity reactions may occur upon exposure to a single or multiple milk protein(s). Numerous attempts have been made to reduce or eliminate the allergenicity of milk proteins. Of these attempts, most have focussed on two approaches: to alter the structure and property of milk proteins through thermal treatments, biochemical processes (enzymatic digestion), irradiation [
3] and high pressure treatments [
4], and to modulate immune responses through sensitization and tolerance induction by means of controlled exposure to a specific allergen which is commonly referred to as specific immunotherapy [
5]. Nevertheless, total avoidance of cow's milk or its associated products still remains as the best remedy for CMA. Hypersensitivity to orally ingested food usually occurs upon failure to induce oral tolerance. Research with germ-free mice has indicated that the interaction between allergens and host's gut microbiota plays a crucial role in oral tolerance development [
6] and in reducing secretions of allergen-specific antibodies [
7]. The gut microbiota is also reported to favour anti-allergenic reactions by mediating T-helper-1 (Th1) type of immunity [
8] or inducing IL-10 and transforming growth factor-β (TGF-β) that suppresses T-helper-2 (Th2) type of immunity [
9]. Recently, delayed microbial exposure and/or reduced diversity of the gut microbiota among children have been associated with higher allergy incidences [
10]. This concept was first reported by Strachan [
11] and later widely known as the 'hygiene hypothesis'. Interestingly, whereas the gut microbiota of allergic infants contained higher levels of
Clostridia, intestinal
Lactobacilli and
Bifidobacteria were more predominant among healthy infants [
12,
13]. Such findings have triggered considerable scientific interests in probiotics, particularly
Lactobacilli and
Bifidobacteria, for prevention or treatment of allergies among infants. The allergy reducing effects of probiotics against food allergens such as egg ovalbumin [
14,
15] and whey proteins [
16] have been demonstrated in mouse allergy models. But, to the best of our knowledge, probiotic effects of
Lactobacillus rhamnosus GG (LGG) to reduce or control allergy to whole cow's milk protein (CMP) have not yet been reported in a mouse allergy model. We used the Balb/C mice model based on its similarity with the human immune system, particularly the Th1 and Th2 responses [
17].
Oral sensitization is well recognized as an ideal route to investigate allergic responses to food allergens. Because mice usually develop oral tolerance and fail to manifest allergic responses to ingested allergens, allergens are frequently co-administered with an adjuvant. However, recent reports indicate that commonly used adjuvants, such as cholera toxin (CT) and alum, possess immune-stimulatory properties that may falsely test non-allergenic food products as positive [
18]. Consequently, there is increasing interest to develop adjuvant-free systemic sensitization models for testing food allergenicity in mice. The main objectives of this study were to evaluate probiotic effects of LGG on CMA development in a Balb/C mouse model using either an adjuvant-assisted oral sensitization (CMP with cholera toxin B-subunit, CTB) method or an adjuvant-free systemic sensitization (CMP only) method.