The existence of alternative memory cells does not imply that they are essential, or important at all, to a healthy immune system. Evolution is not omnipotent; it does the best it can with what it has to work with, and the results are not always elegant, streamlined, or ideal. This is evidenced by the maintenance of alleles that will result in harm to a small population but help a larger one (285); by the continued existence of vestigial organs (286); and by the occasional reappearance of ancestral traits (287). It is therefore possible that alternative memory cells are more a quirk of biology than a useful component of the immune system.
Alternative memory cells may be, for example, an evolutionary relic, a remnant of the early history of adaptive immunity. It may be that alternative memory CD8+ T cells once did play an important role in immune response – perhaps through the potential functions discussed above – but that, over time, conventional T-cell responses evolved to be more effective, better coordinated, or longer-lived, and there is no longer a need for alternative memory cells. Though this cannot be proved conclusively, to explore this hypothesis further it would be instructive to determine whether alternative memory cells also exist in lower vertebrates, and if so, whether they comprise a larger percentage of total T cells in these species than in mice.
The mechanisms by which alternative memory CD8+ T cells appear to arise are known to be important to other aspects of immunity. Specifically, IL-7 and IL-15, along with weak TCR stimulation by self-peptide ligands, are important for T-cell homeostasis. The ability of IL-2 and other cytokines to act as costimulatory factors may augment T-cell responses to inflammatory stimuli. Together, these properties of the immune system
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may result in the occasional generation of an alternative memory CD8+ T cell due to stochastic, localized cytokine increases. Under this view, alternative memory cells are essentially a byproduct of processes that primarily function to promote T-cell
homeostasis. Of course, we can never be sure that a biological phenomenon evolved solely as a byproduct of something else rather than based on its own merits, just as we can never really know “why” anything evolved as it did. All we can do is look for
satisfying explanations.
It’s important to note that, even if alternative memory is unimportant for normal immune responses, its study may still be worthwhile. First, because conventional and alternative memory cells are nearly indistinguishable, characterizing the latter may improve our understanding of the former. Second, as anyone with appendicitis can attest to, some aspects of our biology are mostly dispensable when functioning normally but, under the right conditions, can be the source of serious morbidity. In the next section, we discuss this potential aspect of alternative memory T cells.
What role might alternative memory cells play in disease?
Alternative memory CD8+ T cells and IBD
Inflammatory bowel disease is characterized by chronic inflammation and immune disregulation in the gastrointestinal tract (91). Although often thought of as driven primarily by CD4+ T cells, IBD can also be induced by CD8+ T cells in mouse models (288, 289). A common method to induce colitis experimentally involves the transfer of either naïve CD4+ or naïve CD8+ T cells into lymphopenic mice, typically mice lacking Rag1 or Rag2. After transfer, both types of T cells undergo rapid spontaneous
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proliferation that is dependent on IL-6 and an intact microbiota (288, 290). However, it may possible for elevated levels of cytokines such as IL-2 or IL-4 to substitute for microbial stimulation. Thus, in mice with intrinsic defects in cytokine production, such as Ndfip1KO mice, T cells may require little stimulation from foreign antigen to take on a memory phenotype. Such a possible effect of increased IL-4 may explain the inability of antibiotic treatment to reduce T-cell activation and intestinal pathology in our study (Chapter 2). This hypothesis could be tested by attempting to block IL-4 in vivo with
antibody in Ndfip1 cKO mice with or without concurrent antibiotic treatment. As the balance of intestinal cytokines is an important factor in the pathology of IBD (291), it would also be interesting to test this hypothesis in IL-10 deficient mice, which develop spontaneous colitis unless treated with antibiotics or raised in germ-free housing (97). If germ-free IL-10KO mice given exogenous IL-4 develop colitis, it would suggest that sufficient inflammatory cytokine levels can drive IBD pathology even with minimal microbial stimulation, possibly through spontaneous proliferation of colitogenic T cells. This may explain the epidemiological finding that IBD and asthma, a disease typically characterized by increased IL-4+ T cells, are often co-morbid (292–294) (Figure 15).
In addition to directly promoting an increase in memory-phenotype cells, common γ chain cytokines may contribute to IBD by promoting increased IL-6 production, which is a common feature in IBD (291). Mice with a truncated form of the common γ chain receptor spontaneously develop colitis driven by IL-6+ T cells; loss of this receptor has been observed to lead to increased IL-2 and IL-15 levels (155, 295). IL-4 can induce IL-6 production in keratinocytes, and we have observed an IL-4-dependent increase in serum IL-6 in Ndfip1KO mice (206, 296). Elevated levels of IL-6 may contribute to increased T- cell activation (221).
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CD8+ T cells may also contribute to IBD in humans. Patients with IBD have elevated percentages of peripheral CD4+ and CD8+ T cells with an activated phenotype (297). Peripheral CD8+ T cells isolated from patients with IBD are significantly more likely to produce IFNγ in response to stimulation by co-culture with epithelial cells than T cells from healthy controls (298). This was not observed in CD4+ T cells and was dependent on MHC I. Whether this property of IBD-associated CD8+ T cells contributes to disease etiology or whether it arises as a byproduct of inflammation remains to be characterized. Additionally, CD8+ T cells from human patients with IBD can be divided based on
transcriptional profiling into 2 subsets that are associated with different prognoses, with the subset expressing genes signatures associated with IL-2, IL-7, and TCR signaling being predictive of more severe or persistent IBD (299). Further, this transcriptional signature was extremely similar to that observed in CD8+ T cells from patients with autoimmune disease (300).
Alternative memory CD8+ T cells and allergy
As discussed in Chapter 1, IL-4 plays an important role in establishing and maintaining pathological responses in many examples of allergy. One can imagine two potential roles for alternative memory CD8+ T cells in allergy: first, they might contribute to pathology, possibly through direct tissue damage by nonspecific cytotoxic activity. Second, these T cells might instead help mitigate disease, perhaps by production of IFNγ (Figure 15). Evidence exists for both possibilities.
In mouse models of allergic airway disease, CD8+ T cells that develop in the presence of IL-4+ CD4+ cells worsen lung pathology (301, 302). Additionally, increased bronchial CD8+ cell infiltrate in human asthma patients is predictive of decreased lung function
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(303). Moreover, CD8+ T cells are involved in some types of allergic contact dermatitis and cause apoptosis of skin cells (17, 304–306). A subset of these CTLs found in nickel allergies can lyse nickel-containing cells independent of MHC interactions (307, 308). To test the ability of alternative memory to mediate allergic reactions such as contact
dermatitis, alternative memory CD8+ T cells could be transferred to recipients who are then sensitized and challenged with allergen. As discussed previously, such experiments may establish sufficiency but not necessity; without a method for specifically depleting these cells, it is difficult to determine whether alternative memory cells are necessary for the development of the allergic reactions in vivo.
A role for CD8+ T cells in inhibiting allergic responses has also been observed. In asthma models, although CD8+ T cells can contribute to pathology, it has been suggested that this may be due to Tc2-polarized IL-13+ CD8+ T cells and that Tc1- polarized CD8+ T cells may actually ameliorate disease (309–311). Remarkably, there is already indirect evidence that alternative memory CD8+ T cells can mediate this
protection. Mice which transgenically overexpress IL-15 have an increased memory-like CD8+ T-cell population, and these T cells were necessary and sufficient to suppress eosinophilia and Th2-type cytokine production (312). A similar finding is observed in adult mice that had been immunized shortly after birth with semi-allogeneic dendritic cells (302). TCR transgenic CD8+ T cells activated
in vitro, then transferred to mice
intranasally, were able to prevent airway eosinophilia in a model of airway inflammation through antigen-nonspecific IFNγ production (313). Thus, it is possible that alternative memory cells which develop due to allergy-induced IL-4 may actually serve to limit the extent of pathology.
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Concluding remarks
In this work, we have examined the influence of two external factors, intestinal bacteria and IL-4, on the phenotype of CD8+ T cells. We conclude that IL-4, but not gut bacteria, is necessary for the increase in memory-phenotype CD8+ T cells in Ndfip1-deficient mice. We have discussed the potential implications of this finding for the role of CD8+ T cells in health and disease. Our work has reinforced the idea that excess production of a single cytokine can have manifold effects on the immune system and the health of the host.
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REFERENCES
1. Fåhraeus, R. 1929. THE SUSPENSION STABILITY OF THE BLOOD. Physiol. Rev.
9: 241–274.
2. Kushner, I. 2013. The 4 Humors and Erythrocyte Sedimentation: The Most Influential Observation in Medical History. Am. J. Med. Sci. .
3. Fernel, J., J. M. Forrester, and J. Henry. 2003. The “Physiologia” of Jean Fernel (1567). Trans. Am. Philos. Soc. 93: i–iii, v–xv, xvii–xviii, 1–601, 603–636.
4. West, C. E., D. J. Videky, and S. L. Prescott. 2010. Role of diet in the development of immune tolerance in the context of allergic disease. Curr. Opin. Pediatr. 22: 635–41.
5. Maslowski, K. M., and C. R. Mackay. 2011. Diet, gut microbiota and immune responses. Nat. Immunol. 12: 5–9.
6. Brenner, I. K., J. W. Castellani, C. Gabaree, A. J. Young, J. Zamecnik, R. J. Shephard, and P. N. Shek. 1999. Immune changes in humans during cold exposure: effects of prior heating and exercise. J. Appl. Physiol. 87: 699–710.
7. LaVoy, E. C. P., B. K. McFarlin, and R. J. Simpson. 2011. Immune responses to exercising in a cold environment. Wilderness Environ. Med. 22: 343–51.
8. Walsh, N. P., M. Gleeson, R. J. Shephard, M. Gleeson, J. A. Woods, N. C. Bishop, M. Fleshner, C. Green, B. K. Pedersen, L. Hoffman-Goetz, C. J. Rogers, H. Northoff, A. Abbasi, and P. Simon. 2011. Position statement. Part one: Immune function and exercise. Exerc. Immunol. Rev. 17: 6–63.
9. Follenweider, L. M., and A. Lambertino. 2013. Epidemiology of asthma in the United States. Nurs. Clin. North Am. 48: 1–10.
10. De Vries, J. E., J. Punnonen, B. G. Cocks, R. de Waal Malefyt, and G. Aversa. 1993. Regulation of the human IgE response by IL4 and IL13. Res. Immunol. 144: 597–601.
11. Shimoda, K., J. van Deursen, M. Y. Sangster, S. R. Sarawar, R. T. Carson, R. A. Tripp, C. Chu, F. W. Quelle, T. Nosaka, D. A. Vignali, P. C. Doherty, G. Grosveld, W. E. Paul, and J. N. Ihle. 1996. Lack of IL-4-induced Th2 response and IgE class switching in mice with disrupted Stat6 gene. Nature 380: 630–3.
12. Metzger, H., G. Alcaraz, R. Hohman, J. P. Kinet, V. Pribluda, and R. Quarto. 1986. The receptor with high affinity for immunoglobulin E. Annu. Rev. Immunol. 4: 419–70.
13. Jones, B. L., and G. L. Kearns. 2011. Histamine: new thoughts about a familiar mediator. Clin. Pharmacol. Ther. 89: 189–97.
14. O’Donnell, M. C., S. J. Ackerman, G. J. Gleich, and L. L. Thomas. 1983. Activation of basophil and mast cell histamine release by eosinophil granule major basic protein. J. Exp. Med. 157: 1981–91.
15. Kouro, T., and K. Takatsu. 2009. IL-5- and eosinophil-mediated inflammation: from discovery to therapy. Int. Immunol. 21: 1303–9.
84
16. Desvignes, C., N. Etchart, J. Kehren, I. Akiba, J. F. Nicolas, and D. Kaiserlian. 2000. Oral administration of hapten inhibits in vivo induction of specific cytotoxic CD8+ T cells mediating tissue inflammation: a role for regulatory CD4+ T cells. J. Immunol. 164:
2515–22.
17. Vocanson, M., A. Hennino, M. Cluzel-Tailhardat, P. Saint-Mezard, J. Benetiere, C. Chavagnac, F. Berard, D. Kaiserlian, and J.-F. Nicolas. 2006. CD8+ T Cells Are Effector Cells of Contact Dermatitis to Common Skin Allergens in Mice. J Invest Dermatol 126:
815–820.
18. Saint-Mezard, P., F. Berard, B. Dubois, D. Kaiserlian, and J.-F. Nicolas. The role of CD4+ and CD8+ T cells in contact hypersensitivity and allergic contact dermatitis. Eur. J. Dermatol. 14: 131–8.
19. Van Stipdonk, M. J., E. E. Lemmens, and S. P. Schoenberger. 2001. Naïve CTLs require a single brief period of antigenic stimulation for clonal expansion and
differentiation. Nat. Immunol. 2: 423–9.
20. Kaech, S. M., and R. Ahmed. 2001. Memory CD8+ T cell differentiation: initial antigen encounter triggers a developmental program in naïve cells. Nat. Immunol. 2:
415–22.
21. Blair, D. A., D. L. Turner, T. O. Bose, Q.-M. Pham, K. R. Bouchard, K. J. Williams, J. P. McAleer, L. S. Cauley, A. T. Vella, and L. Lefrançois. 2011. Duration of antigen availability influences the expansion and memory differentiation of T cells. J. Immunol.
187: 2310–21.
22. Sadegh-Nasseri, S., S. K. Dalai, L. C. Korb Ferris, and S. Mirshahidi. 2010. Suboptimal engagement of the T-cell receptor by a variety of peptide-MHC ligands triggers T-cell anergy. Immunology 129: 1–7.
23. Mueller, D. L. 2004. E3 ubiquitin ligases as T cell anergy factors. Nat. Immunol. 5:
883–90.
24. Curtsinger, J. M., D. C. Lins, and M. F. Mescher. 2003. Signal 3 determines tolerance versus full activation of naive CD8 T cells: dissociating proliferation and development of effector function. J. Exp. Med. 197: 1141–51.
25. Valenzuela, J., C. Schmidt, and M. Mescher. 2002. The roles of IL-12 in providing a third signal for clonal expansion of naive CD8 T cells. J. Immunol. 169: 6842–9.
26. Kolumam, G. A., S. Thomas, L. J. Thompson, J. Sprent, and K. Murali-Krishna. 2005. Type I interferons act directly on CD8 T cells to allow clonal expansion and memory formation in response to viral infection. J. Exp. Med. 202: 637–50.
27. Curtsinger, J. M., C. M. Johnson, and M. F. Mescher. 2003. CD8 T cell clonal expansion and development of effector function require prolonged exposure to antigen, costimulation, and signal 3 cytokine. J. Immunol. 171: 5165–71.
28. Williams, M. A., and M. J. Bevan. 2007. Effector and memory CTL differentiation.
85
29. Kedzierska, K., S. A. Valkenburg, P. C. Doherty, M. P. Davenport, and V. Venturi. 2012. Use it or lose it: establishment and persistence of T cell memory. Front. Immunol.
3: 357.
30. Kaech, S. M., J. T. Tan, E. J. Wherry, B. T. Konieczny, C. D. Surh, and R. Ahmed. 2003. Selective expression of the interleukin 7 receptor identifies effector CD8 T cells that give rise to long-lived memory cells. Nat. Immunol. 4: 1191–8.
31. Ibegbu, C. C., Y.-X. Xu, W. Harris, D. Maggio, J. D. Miller, and A. P. Kourtis. 2005. Expression of Killer Cell Lectin-Like Receptor G1 on Antigen-Specific Human CD8+ T Lymphocytes during Active, Latent, and Resolved Infection and its Relation with CD57.
J. Immunol. 174: 6088–6094.
32. Joshi, N. S., W. Cui, A. Chandele, H. K. Lee, D. R. Urso, J. Hagman, L. Gapin, and S. M. Kaech. 2007. Inflammation directs memory precursor and short-lived effector CD8(+) T cell fates via the graded expression of T-bet transcription factor. Immunity 27:
281–95.
33. Hand, T. W., M. Morre, and S. M. Kaech. 2007. Expression of IL-7 receptor alpha is necessary but not sufficient for the formation of memory CD8 T cells during viral
infection. Proc. Natl. Acad. Sci. U. S. A. 104: 11730–5.
34. Sarkar, S., V. Kalia, W. N. Haining, B. T. Konieczny, S. Subramaniam, and R. Ahmed. 2008. Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates. J. Exp. Med. 205: 625–40.
35. Huster, K. M., V. Busch, M. Schiemann, K. Linkemann, K. M. Kerksiek, H. Wagner, and D. H. Busch. 2004. Selective expression of IL-7 receptor on memory T cells identifies early CD40L-dependent generation of distinct CD8+ memory T cell subsets.
Proc. Natl. Acad. Sci. U. S. A. 101: 5610–5.
36. Takemoto, N., A. M. Intlekofer, J. T. Northrup, E. J. Wherry, and S. L. Reiner. 2006. Cutting Edge: IL-12 inversely regulates T-bet and eomesodermin expression during pathogen-induced CD8+ T cell differentiation. J. Immunol. 177: 7515–9.
37. Rao, R. R., Q. Li, K. Odunsi, and P. A. Shrikant. 2010. The mTOR kinase determines effector versus memory CD8+ T cell fate by regulating the expression of transcription factors T-bet and Eomesodermin. Immunity 32: 67–78.
38. Intlekofer, A. M., N. Takemoto, E. J. Wherry, S. A. Longworth, J. T. Northrup, V. R. Palanivel, A. C. Mullen, C. R. Gasink, S. M. Kaech, J. D. Miller, L. Gapin, K. Ryan, A. P. Russ, T. Lindsten, J. S. Orange, A. W. Goldrath, R. Ahmed, and S. L. Reiner. 2005. Effector and memory CD8+ T cell fate coupled by T-bet and eomesodermin. Nat. Immunol. 6: 1236–44.
39. Ichii, H., A. Sakamoto, M. Hatano, S. Okada, H. Toyama, S. Taki, M. Arima, Y. Kuroda, and T. Tokuhisa. 2002. Role for Bcl-6 in the generation and maintenance of memory CD8+ T cells. Nat. Immunol. 3: 558–63.
86
40. Ichii, H., A. Sakamoto, Y. Kuroda, and T. Tokuhisa. 2004. Bcl6 acts as an amplifier for the generation and proliferative capacity of central memory CD8+ T cells. J. Immunol.
173: 883–91.
41. Kallies, A., A. Xin, G. T. Belz, and S. L. Nutt. 2009. Blimp-1 transcription factor is required for the differentiation of effector CD8(+) T cells and memory responses.
Immunity 31: 283–95.
42. Rutishauser, R. L., G. A. Martins, S. Kalachikov, A. Chandele, I. A. Parish, E. Meffre, J. Jacob, K. Calame, and S. M. Kaech. 2009. Transcriptional repressor Blimp-1
promotes CD8(+) T cell terminal differentiation and represses the acquisition of central memory T cell properties. Immunity 31: 296–308.
43. Brenchley, J. M., N. J. Karandikar, M. R. Betts, D. R. Ambrozak, B. J. Hill, L. E. Crotty, J. P. Casazza, J. Kuruppu, S. A. Migueles, M. Connors, M. Roederer, D. C. Douek, and R. A. Koup. 2003. Expression of CD57 defines replicative senescence and antigen-induced apoptotic death of CD8+ T cells. Blood 101: 2711–20.
44. Lee, J.-B., and J. Chang. 2010. CD43 Expression Regulated by IL-12 Signaling Is Associated with Survival of CD8 T Cells. Immune Netw. 10: 153–63.
45. Hikono, H., J. E. Kohlmeier, S. Takamura, S. T. Wittmer, A. D. Roberts, and D. L. Woodland. 2007. Activation phenotype, rather than central- or effector-memory
phenotype, predicts the recall efficacy of memory CD8+ T cells. J. Exp. Med. 204: 1625–
20070322.
46. Stonier, S. W., L. J. Ma, E. F. Castillo, and K. S. Schluns. 2008. Dendritic cells drive memory CD8 T-cell homeostasis via IL-15 transpresentation. Blood 112: 4546–54.
47. Stemberger, C., K. M. Huster, M. Koffler, F. Anderl, M. Schiemann, H. Wagner, and