What kind of tissues harbor immune cells

The defining feature of tissue-resident lymphocytes is their distinct migration pattern. This blood-tissue disequilibrium can be conveniently approximated by intravascular staining 8 — Intravenous administration of fluorescently-conjugated antibody labels vasculature-associated cell populations in a short period of time. Unlabeled cells are thus presumed to reside in the tissue parenchyma and are unlikely to re-circulate. The tissue resident property is most formally demonstrated by parabiosis experiments in which the circulatory systems of two animals are surgically joined, allowing for free exchange of their cell populations Over time, half of the re-circulating lymphocyte compartment in one animal will be derived from its parabiont 6 , In contrast, the non-circulating compartment remains dominated by endogenous lymphocyte populations with little to no input from the parabiont 6 , This restricted migratory pattern of tissue-resident lymphocytes is often associated with their lack of lymphoid tissue homing chemokine receptors and elevated expressions of several adhesion molecules 7 , In addition, increased expression of integrin molecules, such as CD49a encoded by Itga1 and CD encoded by Itgae , whose ligands are collagen and E-cadherin, respectively, promotes interaction with tissue constituents, further reinforcing retention of lymphocytes 18 , Whereas, the downregulation of CCR7 and S1PR1 seems to be universal for tissue-resident lymphocytes, the usage of integrin molecules is more diverse.

CD is specifically found on lymphocytes associated with epithelial tissues, such as the small intestine epithelium and ductal epithelium in glandular organs 20 — CD49a and CD69 also have their own tissue-restricted expression patterns 18 , 24 — These observations highlight the substantial heterogeneity within the tissue-resident lymphocyte compartment.

Thus, defining tissue-resident populations solely based on phenotypic markers may not reliably identify all cells. Instead, parabiosis experiments remain the gold standard to properly define tissue residency. So far, tissue-resident populations have been identified for all known types of lymphocyte across the innate-adaptive spectrum 6 , strongly suggesting that the acquisition of the tissue residency program represents a state of differentiation rather than commitment to a distinct lineage.

Resident lymphocyte populations are hypothesized to sense in their home organs tissue disturbances stemming from infection, stress and other deviations from the norm. In turn, they initiate the necessary immune responses to restore homeostasis. Below we briefly describe the characteristic features of various tissue-resident lymphocyte populations and their functions in maintaining tissue integrity. Innate lymphocytes are characterized by their lack of functionally re-arranged antigen receptors.

This population includes the prototypic member, natural killer NK cells, and the emerging family of innate lymphoid cells ILCs 27 , The killer ILCs, on the other hand, are mostly found in the liver and epithelium of glandular tissues, such as the salivary, prostate, and mammary glands, and can mediate direct cytolysis of target host cells through granzyme secretion or Fas ligand engagement 23 , 29 — The exact function of tissue-resident type 1 innate lymphocytes remains contentious.

Because of their striking resemblance to NK cells at the phenotypic level, studies aiming to test NK cell functions by depleting NK marker-expressing populations through antibodies or diphtheria toxin system may have inadvertently eliminated type 1 ILCs as well. Hence it is difficult to pinpoint which population mediates the observed phenotypes.

This caveat has only been recognized recently but nevertheless precipitated the development of new genetic tools to selectively target either populations. For instance, a recent study utilized animals deficient for the transcription factor Zfp , or Hobit , to specifically reduce the number of liver ILCs, leaving the NK compartment intact In these animals, control of early viral replication in the liver was impaired, supporting the idea that resident type 1 ILCs function as first line defenders.

Type 2 ILCs are the most homogenous among the innate lymphocytes and produce signature cytokines of the type 2 response, such as IL-5, IL, and amphiregulin, in a transcription factor Gata3 -, Bcl11b , and Rora -dependent manner 33 — ILC2s control normal immune responses through cross-talk between stroma and other immune cell types.

For instance, during helminth infection, intestinal tuft cell-derived IL activates ILC2s to secrete IL, which feedbacks on the epithelium to promote tuft cell differentiation The alarmin IL produced upon tissue injury also stimulates IL-5 production by ILC2s, which in turn recruits eosinophils and enhance their innate effector functions This pathway can be antagonized by a secretory product of the helminth H. Group 3 ILCs are highly complex and can be roughly unified by their dependency on the transcription Rorc for development and function Upon activation by IL, a subset of ILC3s produce IL, which in turn triggers the anti-microbial peptide production by intestinal epithelium 40 — Furthermore, IL in concert with IL is essential for control of murine norovirus infection Together, these data demonstrate a critical role for ILC3s in maintaining gut homeostasis.

Innate-like or unconventional T cells express functionally re-arranged T cell receptors TCRs of limited diversity. In contrast to conventional T cells whose TCRs strictly recognize peptides in the context of classical polymorphic major histocompatibility molecules MHCs , the mode of antigen recognition by innate-like T cells is diverse, with TCRs recognizing antigen in the context of canonical MHCs, non-classical non-polymorphic MHC-like molecules, or even independently of MHCs altogether Many epithelial tissues contain resident IEL populations These include the Ly49 and other NK receptor family members 49 — So far, the exact functions of IELs remain elusive, although in specific settings, IELs contribute to anti-pathogen responses in the gut 52 , 53 , 55 , The synthetic glycolipid, alpha-galactosylceramide, has been one of the prototypic stimulators of iNKT cells Since then, a plethora of structurally homologous lipids capable of activating iNKT cells have been identified These range from foreign substances, such as certain bacterial cell wall components 63 — 65 to endogenous sources, such as intermediates in lipid metabolism 66 , 67 , although the latter is often only transiently present, rare, and less potent.

Nevertheless, sensing of endogenous lipid ligands may be the major mechanism by which iNKT cells detect a breach of tissue integrity. Two studies demonstrate an essential role for iNKT cells in controlling infection by pathogens that lack potent agonist ligands 68 , 69 , supporting the idea that iNKT cells may primarily survey host cells for altered metabolism as a result of pathogen invasion. Not unlike these T helper cells, each iNKT cell subset produces its signature cytokines driven by distinct master transcription factors The term tissue-resident memory T cells specifically describe populations of conventional T cells that acquire tissue-resident properties.

T RM cells have been commonly regarded as first line of defense in peripheral tissues especially against previously encountered threats 79 — They are hypothesized to provide timely control of tissue threats before the participation of circulatory memory populations.

For instance, a report showed that pre-existing herpes simplex virus HSV 2 antigen-specific T RM cells at the vaginal mucosa protect hosts from lethal HSV-2 challenge by restricting viral replication at the site of infection as well as preventing the spread of virus to the peripheral nervous system T RM cells engage in diverse effector functions to mediate host protection.

Notably, T RM cells in the brain can lyse antigen-loaded targets in situ 84 , suggesting their cytotoxic potential and direct killing as their means of immunosurveillance. More strikingly, recent studies highlighted the innate-like effector property of T RM cells 83 , 86 , Local activation of T RM cells resulted in their chemokine production, which potently recruited non-antigen specific T cells and initiated an innate immune cascade.

Such a bystander response resulted in near-sterilizing immunity against antigentically unrelated pathogens. Thus, in this context, T RM cells can serve as alarm-sounders rather than front line defenders.

Adaptive lymphocytes are naturally circulatory and only acquire tissue residency program upon activation. In contrast, innate and innate-like lymphocytes migrate directly to their home tissues after exiting sites of development, bypassing this recirculatory step. The developmental pathway of thymocytes to mature T cells is punctuated by several checkpoints, one of which occurs at the double-positive DP stage Figure 1.

Strong self-reactivity instructs DP thymocytes to adopt innate-like T cells fates whereas weakly reactive clones are diverted into conventional T cell lineages For instance, thymocytes expressing a transgenic TCR predominantly develop into unconventional IELs when its cognate ligand is expressed in the thymus, but into conventional T cells when otherwise. This process of agonist selection instructs a phenotypic change on DP thymocytes characterized by the downregulation of both CD4 and CD8 co-receptors and the concomitant upregulation of PD-1 89 — Thus, strong self-reactivity underlies the innate-like T cell fate choice.

Figure 1. Ontogeny of tissue-resident lymphocytes. All lymphocytes develop from the common lymphoid progenitor CLP. Whether NK cells can acquire tissue-resident features remains unknown. Thus, the term tissue-resident NK trNK cells is better kept until such a possibility can be unequivocally ruled out.

Beside innate lymphocytes, CLP also gives rise to T lineage-committed progenitors that complete their differentiation in the thymus. DP thymocytes bearing strongly self-reactive TCRs develop into unconventional intraepithelial lymphocyte IEL and natural killer T NKT cell lineages through agonist selection, while those with weakly self-reactive TCRs are diverted into single positive SP thymocytes, which subsequently give rise to conventional T conv.

T cells. Because innate lymphocytes do not express antigen receptors, their self-reactivity is difficult to gage. However, there exist several striking parallels between innate lymphocyte and T cell development. All innate lymphocytes appear to arise from an early innate lymphoid progenitor EILp; Figure 1. One defining feature of EILp is downregulation of IL-7 receptor IL-7R , which also occurs in DP thymocytes, presenting a peculiar similarity between the two progenitors among the otherwise IL-7R-dependent intermediates during lymphopoeisis 96 , The developmental path of cytotoxic ILCs is less understood.

In contrast to IL-7R-expressing helper ILCs, which require the transcription factor Gata3 and Nfil3 for development, cytotoxic ILCs in the salivary gland are marginally affected upon loss of either transcription factors 29 , 31 , 99 — These genetic data suggest the existence of yet another innate lymphocyte lineage, which is distinct from both the ILCh and conventional NK cells, and is tentatively named ILCk Figure 1.

ILCks in fact resemble IEL in their constitutive expression of cytotoxic molecules and inherent tissue-resident nature Best exemplified by T RM cells, re-circulating lymphocytes can acquire tissue resident properties upon activation. The exact time point at which the tissue-resident program is launched during the activation history of a T cell is still unknown.

Several lines of evidence suggest that tissue tropism of an activated T cells can be imprinted by dendritic cells DCs during priming. Furthermore, the expression of skin- and gut-homing receptors can be enhanced by metabolites specific to these two tissues, such as retinoic acid , These data collectively suggest that activated T cells acquire tissue tropism and specific homing capacity during priming.

Contrary to this model, recent studies demonstrated that T cell migration is rather promiscuous during the effector phase of the immune response. In fact, T cells primed at any site can access almost every tissue in the organism. For instance, priming of T cells during systemic LCMV infection leads to the migration of antigen-specific T cells to many peripheral tissues More strikingly, intranasal immunization with Sendai virus also results in the migration of antigen-specific T cells to other peripheral tissues Further examination revealed that T cells primed in any secondary lymphoid organs can in fact upregulate homing receptors for non-lymphoid tissues Thus, the entry of a T cell into non-lymphoid tissues can be instated regardless of priming locations.

Once inside the tissue, local signals then orchestrate the tissue resident program. These data suggest that entry into the tissue is a stochastic but pivotal event that marks the initiation of tissue resident program. The discrepancy may be caused by the use of different infection models. Interestingly, although both KLRG1-fate mapped and non-fate mapped precursors lost KLRG1 expression when entering the tissue, the progeny of the two exhibits nuanced but discernable differences in effector functions , suggesting that other events before tissue entry can impact the functional capacity of T RM.

Often deemed as the counterpart to conventional CD8 T cells, whether NK cells can acquire tissue resident features like T RM differentiation is less understood. These results suggest that re-circulating conventional NK cells possess the tissue resident potential, but its manifestation requires tissue-specific signals.

Further studies, such as fate-mapping experiments, are needed to formally test this hypothesis. Long-term parabiosis experiments revealed that under steady-state conditions, tissue resident lymphocytes are long-lived and replenish their population predominantly by local expansion 6. Consistently, other studies in mice and rhesus macaques showed that the tissue memory CD8 T cell populations are stable for — days, with little to no input from the circulatory memory pool — These observations suggest that while the concerted actions of adhesion molecules and chemokine receptors enforce tissue retention, additional cell-extrinsic signals promote the maintenance of tissue resident lymphocytes.

T RM in the non-lymphoid tissues, such as the skin, are critically dependent on IL 18 whereas those in the secondary lymphoid organs are not While T RM are induced in an antigen-dependent manner, they can be maintained in the absence of cognate antigen in the skin, reproductive tract, and salivary glands 18 , 19 , In other tissues, persisting antigens contribute to T RM differentiation 19 , 26 , 82 , 84 , Thus, the requirement for antigen during T RM maintenance may be tissue-specific.

Lastly, given the similar requirement for IL-7 and IL during their homeostasis, resident lymphocytes may occupy overlapping tissue niche.

Pinpointing the source of these cytokines in the tissue may help elucidate the redundant and non-redundant roles of each resident lymphocyte population in maintaining tissue integrity. The vertebrate immune system has evolved to exquisitely distinguish self from non-self, thereby achieving effective anti-pathogen responses while curbing autoreactivity.

Cancer presents a unique challenge to this fine-tuned system as transformed cells are pathogenic agents derived from the host itself. Yet prevailing evidence has demonstrated that the immune system exerts constant pressure on tumors These observations underlie the preponderant concept of cancer immunosurveillance — Mechanistically, increased somatic mutation as a result of genomic instability in transformed cells may generate neo-epitopes that can be recognized by conventional adaptive lymphocytes , Targeting this mode of immunosurveillance certainly has been fruitful.

However, not all cancer types sustain high mutation burden , In such cases, CD8 T cell responses elicited by unmutated self-antigen often fail to restrict tumor growth , These findings thus highlight the need to explore other immunosurveillance mechanisms for effective cancer immunotherapies. Figure 2. Cancer immunosurveillance by tissue-resident lymphocytes.

Spontaneous oncogene-driven breast tumors are infiltrated by group 1 innate lymphocytes, conventional, and unconventional T cells. Despite their cytotoxicity, therapies targeting these tissue-resident populations are lacking while rapid advancement has been made to target conventional NK and T cells. Just as pre-existing T RM populations are essential for restraining previously encountered pathogens, prophylactically induced T RM cells by cancer vaccines provide superior control of tumor growth over re-circulating memory T cells , In fact, the presence of circulating tumor antigen-specific CD8 T cells alone is not sufficient to control tumor growth , , highlighting the potential therapeutic benefit of targeting tissue-resident lymphocytes.

Strategies to enhance the differentiation and maintenance of these vaccine-induced T RM cells may decrease the relapse rate as well as restrict metastasis.

However, prophylactic vaccination with tumor-associated antigen may not always be feasible in clinical settings, as it requires knowing the antigen ahead of time when patients who seek medical attention often have developed tumors already. Notwithstanding, tumorigenesis does naturally elicit tissue-resident lymphocyte responses 23 , — Importantly, a substantial fraction of participating lymphocyte populations appear to have cytotoxic potential 23 , , These include conventional T cells of the CD8 lineage as well as more recently identified unconventional T cells and group 1 innate lymphocytes Figure 2.

Below, we summarize the latest findings on their characterization and potential cancer immunosurveillance functions. Among them, populations expressing tissue-resident markers are abundantly found 23 , , , — These include T cells of both the conventional and unconventional lineages. Our understanding of tissue-resident T cell responses in the context of cancer has only begun to advance in recent years. Much of the foundation is in fact built upon extrapolating observations from T RM cells in infectious settings.

While these studies provide an invaluable conceptual framework to start with, cancer and acute infection differ fundamentally. Tumorigenesis is a continuous process without a defined time course. In contrast to acute infections where the pathogen load peaks and wanes within a week's time, tumor-associated antigen is continuously present and, in most oncogene-driven cancer models, persist until the endpoint of disease. In a sense, tumorigenesis is more analogous to chronic than acute infections.

Indeed, the induction and accumulation of dysfunctional cytotoxic T lymphocytes CTLs by persistent antigen stimulation is a shared feature in both settings In a BF10 mouse melanoma transplantable tumor model, a fraction of antigen-specific tumor-infiltrating CD8 T cells acquired CD69 and CD expression 3 weeks after tumor engraftment Furthermore, administration of blocking antibodies against CD resulted in a slight but significant acceleration in tumor growth , implying a CDdependent cancer immunosurveillance mechanism by these putative tissue-resident tumor-infiltrating lymphocytes TILs.

In recent winters, between 12, and 80, Americans have died from influenza and its complications, and the effectiveness of vaccination decreases as people age. The third project will identify B cell populations that contribute to xenograft rejection, particularly B cells specific for swine leukocyte antigens. Xenografts of pig organs into humans would help the more than 1, people awaiting lifesaving transplants through Legacy of Hope. Nationwide, the number awaiting transplants is more than , The third project will also develop tools to rapidly screen blood of people on the transplant list to identify swine leukocyte antigen reactivity.

Although three well-defined carbohydrates that cause xenograft rejection have been removed from genetically engineered pigs, half of humans still have antibodies to lesser-defined antigens that could lead to transplant rejection. Co-leaders of the first project are John Kearney, Ph. Second project co-leaders are Lund and Troy Randall, Ph. Koopman, M. The third project leader is Joseph Tector, M. More than page application Lund and colleagues first saw the NIH request for applications on Dec.

The due date was the end of March Randall, Lund, Tector and Kearney immediately called all hands on deck in their four labs, putting 20 researchers to work full time on preliminary experiments for the U19 grant application. In three and a half months, they demonstrated their ability to collect donor tissues, isolate good numbers of B cells and antibody-secreting cells from the tissues, and analyze the genetic and molecular details of those cells.

The grant application -- totaling more than pages -- was written in three weeks. Articles by Luster, A. Search for related content. Subject Collections Innate Immunity and Inflammation. Interview Click to see an interview with subject collection editor Tom Misteli. Interview Click to see an interview with subject collection editor Tom Cech. Interview Click to see an interview with subject collection editor Lucy Shapiro.

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