Brain, Behavior, and Immunity 26 (2012) 1224–1225

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Brain, Behavior, and Immunity

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Brief Commentary

Can we really know if a stressor increases or decreases natural killer cell activity?

Shamgar Ben-Eliyahu ⇑The Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, IsraelThe School of Psychological Sciences, Tel Aviv University, Tel Aviv, Israel

The article by Tarr and Sheridan published in this issue (Tarret al., 2012) describes a comprehensive study that provides evi-dence for activation of natural killer (NK) cells following repeatedsocial disruption. This study surpasses most studies addressing theissue of stress and NK activities in terms of quality and robustnessof the empirical work, as well as in the breadth of the indices usedto evaluate NK functions. Still, this study and most others assessingthe impact of stressors on NK functions, including those publishedfrom my laboratory, are limited to an extent that it seems almostimpossible to make a general claim of an ‘‘increase’’ or ‘‘decrease’’in NK activity. Before laying my argumentations, some perspec-tives are warranted.

1. Perspectives

Twenty-thirty years ago, the trend in PNI research was to reportsuppression of immune functions, and there were numerous re-ports of NK suppression by a variety of stressors. At that time, wehardly knew how to count NK cells, and thus our interpretationsof the findings were limited or misleading. Similarly, thirty yearsfrom today, our current high-tech methodologies and understand-ings will be obsolete, and our current findings of ‘‘increased’’ or ‘‘de-creased’’ NK activity (or other immune functions) will be viewed aswe today view ‘‘30-year old science’’. As scientists, we try to orga-nize knowledge in parsimonious, easy-to-understand theories,but nature acts in a more complex manner and independently ofour quest for order, coherence, or simplicity!

Second, some years ago a common reasoning for ‘‘why wouldacute stress suppress immunity’’ was ‘‘to preserve energy so thatthe host will allocate all available resources to fight or flight’’. Iclearly prefer the newer interpretation, presented by Tarr, Sheridanand others, claiming that, in fact, stress activates the immune sys-tem to prepare it for a potential immune challenge resulting froman incident of ‘‘fight or flight.’’ This interpretation makes more evo-lutionary sense, but, again, years ago other interpretations madeperfect sense. So, although my heart would like to endorse the gen-eral suggestion made by Tarr and Sheridan, here are some argu-ments for being cautious, and for the claim that it is almostimpossible to know, in a generic manner, whether a particularstressor increases or decreases NK functions.

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⇑ Tel.: +972 3 640 7266; fax: +972 3 640 9520.E-mail address: shamgar@post.tau.ac.il

2. Numerous immune compartments and complex kinetics

Important aspects of the immune system and its reaction tostressors and pathogens are: (i) the divergence of leukocytes thatsimultaneously exist in different immune compartments and in dif-ferent states of activation, and (ii) the host capacity to quickly re-allocate specific leukocytes to specific organs in order to maximizeimmune competence against localized or diffuse challenges. Giventhis dynamic distribution of leukocytes, to gain complete under-standing of NK functions we need to simultaneously sample NKcells from all immune compartments, especially those that areimportant for ongoing challenges. For example, the most importantNK cell populations, in my view, are those that reside in the liversinusoids (pit cells) (Bahjat et al., 2007) or adhere to the capillariesof the lungs (marginating pulmonary NK cells) (Melamed et al.,2010). These NK cells are the most potent endogenous NK cellsknown today, which, similarly to LAK cells, are capable of destroy-ing autologous tumor cells that are allegedly ‘‘NK-resistant.’’ Strate-gically located in the capillaries, these unique NK cells filter andphysically interact with all circulating aberrant cells, and can lysethem. Unfortunately, these NK cell populations are also the leaststudied, mostly because they are the least approachable. As withany other NK cell population, they redistribute in a complex man-ner under stressful conditions (Goldfarb et al., 2011).

The kinetics of change in relation to a stressor is a critical factor,both at the single cell and whole-organism levels. A stressor (acuteor chronic) changes the distribution pattern of leukocytes along timebetween the different immune compartments in a complex manner– e.g., increasing, decreasing and increasing again specific NK cellsubpopulations of different capacities (Shakhar and Ben-Eliyahu,1998). At the single cell level, epinephrine or prostaglandins wereshown to suppress NK activity while present, but to induce a reboundeffect once removed (Hellstrand et al., 1985), which could be asimportant biologically as the suppressive effects. Thus, withoutcounting and assessing the functional capabilities of all NK cell sub-populations simultaneously in all immune compartments along anextended period of time, the picture of how stress affects the NK cellsystem will remain incomplete. Understanding the in vivo biologicalor clinical significance of these changes is an additional obstacle.

3. Limitations of the ex-vivo approach

The ex-vivo approach used to assess the effects of stress on NKactivities can often be misleading. For example, and as alluded

S. Ben-Eliyahu / Brain, Behavior, and Immunity 26 (2012) 1224–1225 1225

above, the in vivo NK-suppressive effects of epinephrine and pros-taglandins cannot be reflected ex-vivo, as they immediately dissi-pate upon removal of leukocytes from the in vivo hormonal andcellular milieu (Hellstrand et al., 1985). On the other hand, the ef-fects of glucocorticoids, which are mediated through DNA-depen-dent protein synthesis, are long-lasting and gradually develop atthe single cell level. Therefore, such effects are artificially enhancedin the transition to an ex-vivo setting. Thus, in vivo suppression ofNK activity by epinephrine or prostaglandins may actually yield anex-vivo increase in NK cytotoxicity (due to the rebound effect); andan insignificant in vivo effect of glucocorticoids (which can shortlybe overcome by NK cell redistribution), may be miss-representedex-vivo as a suppression. Also, many studies artificially activateNK cells in vitro for �24 h by using Th1-cytokines before testingtheir ex-vivo activity. This approach may clearly conceal thein vivo conditions of NK cells at the time of stress, even though itmay indicate their potential for becoming activated. Last, the com-monly used standard target cell lines to assess NK activity areallogeneic or xenogeneic, and their lysis relies on different mecha-nisms than those used for lysing syngeneic/autogenic targetcells (Lanier, 2005). Even more disturbing, we recently found thatstress and surgery differentially affect the lysing of the standardxenogeneic target lines compared to syngeneic target cells (underreview).

4. Few partial solutions

One approach is to empirically address as many as possible ofthe above obstacles, as indeed elegantly conducted in the studyby Tarr and Sheridan. However, it seems impossible to comprehen-sively address all obstacles. A second solution is to indirectly assesscumulative in vivo levels of NK activity over extended periods oftime. For example, we use a highly sensitive NK-dependentin vivo index of lung-tumor-retention following i.v. administration

of syngeneic tumor cells, and compare the effects of stress betweenNK-intact and NK-depleted rats. This approach allows investigatorsto deduce the role of NK cells and the changes in their activity lev-els in mediating the effects of stressors (Ben-Eliyahu et al., 1996;Goldfarb et al., 2011). Admittedly, there are very few in vivo sys-tems that reflect NK activity in a sensitive manner, and these ap-proaches often reflect NK activity of a particular immunecompartment such as lungs, but not skin or spleen. In humans,where assessment is restricted to circulating NK cells, limitationsof ex-vivo approaches should be carefully considered, and the useof autologous tumor cells in cancer patients should be preferredfor clinical studies.

References

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Ben-Eliyahu, S., Page, G.G., Yirmiya, R., Taylor, A.N., 1996. Acute alcohol intoxicationsuppresses natural killer cell activity and promotes tumor metastasis. Nat. Med.2, 457–460.

Goldfarb, Y., Sorski, L., Benish, M., Levi, B., Melamed, R., Ben-Eliyahu, S., 2011.Improving postoperative immune status and resistance to cancer metastasis: acombined perioperative approach of immunostimulation and prevention ofexcessive surgical stress responses. Ann. Surg. 253, 798–810.

Hellstrand, K., Hermodsson, S., Strannegard, O., 1985. Evidence for a beta-adrenoceptor-mediated regulation of human natural killer cells. J. Immunol.134, 4095–4099.

Lanier, L.L., 2005. NK cell recognition. Annu. Rev. Immunol. 23, 225–274.Melamed, R., Rosenne, E., Benish, M., Goldfarb, Y., Levi, B., Ben-Eliyahu, S., 2010. The

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Shakhar, G., Ben-Eliyahu, S., 1998. In vivo beta-adrenergic stimulation suppressesnatural killer activity and compromises resistance to tumor metastasis in rats. J.Immunol. 160, 3251–3258.

Tarr, A.J., Powell, N.D., Reader, B.F., Bhave, N.S., Roloson, A.L., Carson 3rd, W.E.,Sheridan, J.F., 2012. Beta-adrenergic receptor mediated increases in activationand function of natural killer cells following repeated social disruption. Brain.Behav. Immun. 26, 1226–1238.