While some functionalities of probiotics are considered more generic (Hill et al. 2014), the effect on the immune system is considered to be strain specific (Hill et al. 2014). Moreover, the study of immunological effects in vivo is difficult as currently no specific biomarkers for immune functionality have been validated. This is partly related to the fact that the immune status of each individual subject can differ considerably, based on lifestyle, health status and genetics, and, moreover, may change over time, e.g. related to stress, age or seasonal variation.
If the immune response does not function at maximum efficiency, susceptibility to common infectious diseases will increase. For example, it is well known that the immune response weakens in later life, increasing disease risk (Gomez et al. 2008), and that chronic illness also has a negative effect on the immune system. Furthermore, both psychological (Cohen & Williamson 1991) and physical stress (Gleeson 2007) may cause transient immune depression, linked to increased episodes of the common cold.
When discussing immune effects, the understanding of the mechanisms involved is extremely important as this will help to understand how probiotics can support people that are already healthy. This concept, by definition, is difficult to prove as very large-scale preventive studies need to be executed over an extensive period of time. Therefore, developing mechanistic studies might be one of the few options to avoid these very expensive and very exhaustive clinical trials, although human intervention studies will ultimately be required to confirm the immune effects hypothesized.
Natural Killer Cell Activity
Natural killer (NK) cells are part of the innate immune system. They are large lymphocytes lacking antigen-specific receptors, that target and kill abnormal cells including certain tumour cells and cells infected by viruses. NK cells are therefore key players for the immune system to fight viral infections such as the common cold and flu (Brandstadter & Yang 2011), as well as in the prevention of certain types of cancer (Imai et al. 2000).
Lifestyle and environmental factors, such as obesity and cigarette smoking, are known to reduce NK cell activity. Maintaining or strengthening NK cell activity is therefore considered an important mechanism in support of a functional immune system, and the effect of L. casei Shirota on NK cell activity has been explored in healthy adults (Shida et al. 2017, Harbige et al. 2016, Nagao et al. 2000), older adults (Dong et al. 2013, Takeda & Okumura 2007) and smokers (Morimoto et al. 2005, Reale et al. 2012).
Study: NK Cell Activity
Effect on natural killer cell activity in male cigarette smoking adults - Reale et al. (2012) Brit J Nutr 108(2):308-314
Methods: This double-blind placebo-controlled, randomised study studied the effect of L. casei Shirota on natural killer (NK) cell activity in 72 healthy Italian blue collar male cigarette smoking adults. Subjects were given L. casei Shirota powder, 4 sachets/d, for 3 weeks (n=36) (1010 viable cells/sachet) or a placebo (n=36) for three weeks. Baseline and post-consumption samples of peripheral blood mononuclear cells were taken in order to measure NK cell activity and CD16+ lymphocytes (the latter indicate induction of cytotoxic activity of NK and other immune cells). Importantly, before intake, NK cytotoxic activity was found to be inversely correlated with the number of cigarettes smoked.
Results: L. casei Shirota intake was associated with a significant increase in NK cytotoxic activity as well as an increase in CD16+ cells (both P < 0.001), indicating that the probiotic helped to increase NK activity, which was otherwise lower due to smoking.
Dendritic cell, T-cell and cytokine activity
Dendritic cells (DC) and T-cells (or T-lymphocytes) are an essential part of the immune system, involved in identifying foreign substances such as pathogens and initiating a specific immune response. DC are specialised antigen-presenting cells capable of driving antigen-specific T-cell responses, and they determine whether those T-cell responses will be tolerogenic or immunogenic. Cytokines are cell signalling molecules, produced by a broad range of cells including T-cells, that are key in regulating the immune systems response to both infection and inflammation.
In addition to the immunomodulatory effects of probiotics by the modulation of IgA concentrations, some strains of probiotics will also have positive effects on DC, T-cells and cytokines (Ashraf & Shah 2014), including L. casei Shirota.
In 2011, the hypothesis was raised that L. casei Shirota could induce immunoregulatory properties in DC and T-cells, and possibly divert effector T-cells away from intestinal sites to reduce inflammation (Mann et al. 2011). This has been further explored in inflammatory bowel disease, specifically ulcerative colitis (UC) which is characterised by a dysregulated intestinal immune response to the gut microbiota (Mitsuyama et al. 2008).
Study: Dendritic cells
Conditioning intestinal DCs with L. casei Shirota in UC patients - Mann et al. (2014) Inflamm Bowel Dis 20:2299-2307.
Method: Intestinal DCs were isolated from healthy controls and from patients with active ulcerative colitis (UC) (biopsies were taken from both inflamed and non-inflamed colon in UC patients), and conditioned with L. casei Shirota.
Results: The researchers identified skewed DC subsets in UC patients, specifically the loss of CD103+ lymph-node homing DCs important in generating regulatory T-cells that are anti-inflammatory, and infiltrates of CD11c- (non-myeloid marker) DCs with enhanced expression of Toll-like receptors for bacterial recognition in UC patients. The authors hypothesized this likely contributes to a reduced T-cell stimulatory capacity in the inflamed tissue of the UC colon.
Conditioning intestinal DCs with the probiotic strain L. casei Shirota in UC partially restored their normal function, as indicated by a reduced Toll-like receptor 2/4 expression and by the restoration of their ability to imprint homing molecules on T-cells and to generate IL-22 production by stimulated T-cells. Results suggested that T-cell dysfunction in UC is indeed driven by DCs and that bacterial conditioning of gut DCs can indirectly manipulate T-cell responses with implications for immunomodulatory effects of the commensal microbiota in vivo.
Conclusions: The manipulation of DCs allow DC-specific therapy that may be beneficial in inflammatory bowel disease.
Downregulation of inflammatory responses
Downregulation of inflammatory responses by L. casei Shirota has been shown in other applications and through other mechanisms. The strain was shown to provoke anti-inflammatory and immunoregulatory effects in Peyer’s patches (Chiba et al. 2010). In macrophages stimulated by L. casei Shirota, the IL-12/IL-10 cytokine ratio can vary with the bacterial signals they encounter in the environment (Shida et al. 2009; Shida et al. 2011), an effect also observed in vitro for e.g. the activity of the pro-/ anti-inflammatory transcription factor NFκB (Habil et al. 2012).
More recently, a preliminary study investigating whether the anti-inflammatory properties of L. casei Shirota could reduce the inflammatory index of patients with HIV suffering from persisting inflammation, showed positive effects on T-cells and CD56+ cells. Furthermore, cytokine markers on the serum and mRNA level changed significantly, and inflammation and cardiovascular risk decreased. The researchers concluded that L. casei Shirota may modulate immunological parameters and may therefore be an inexpensive and practical strategy to support the immune function of patients with HIV (Falasca et al. 2015).
The effects of L. casei Shirota on immune profiles and intestinal microbial translocation in children with HIV has also been investigated and the findings suggest that short-term L. casei Shirota consumption is a safe supportive approach with immunological and virological benefits in children with HIV (Ishizaki et al. 2017).