Interleukin-2 (IL-2) is an important mediator in the vertebrate immune system. IL-2 is apotent growth factor that mature T lymphocytes use as a proliferation signal and the production ofIL-2 is crucial for the clonal expansion of antigen-specific T cells in the primary immune response.IL-2 driven proliferation is dependent on the interaction of the lymphokine with its cognatemultichain receptor. IL-2 expression is induced only upon stimulation and transcriptionalactivation of the IL-2 gene relies extensively on the coordinate interaction of numerous inducibleand constitutive trans-acting factors. Over the past several years, thousands of papers have beenpublished regarding molecular and cellular aspects of IL-2 gene expression and IL-2 function. Thevast majority of these reports describe work that has been carried out in vitro. However,considerably less is known about control of IL-2 gene expression and IL-2 function in vivo.
To gain new insight into the regulation of IL-2 gene expression in vivo, anatomical anddevelopmental patterns of IL-2 gene expression in the mouse were established by employing in situhybridization and immunohistochemical staining methodologies to tissue sections generated fromnormal mice and mutant animals in which T -cell development was perturbed. Results from thesestudies revealed several interesting aspects of IL-2 gene expression, such as (1) induction of IL-2gene expression and protein synthesis in the thymus, the primary site of T-cell development in thebody, (2) cell-type specificity of IL-2 gene expression in vivo, (3) participation of IL-2 in theextrathymic expansion of mature T cells in particular tissues, independent of an acute immuneresponse to foreign antigen, (4) involvement of IL-2 in maintaining immunologic balance in themucosal immune system, and (5) potential function of IL-2 in early events associated withhematopoiesis.
Extensive analysis of IL-2 mRNA accumulation and protein production in the murinethymus at various stages of development established the existence of two classes of intrathymicIL-2 producing cells. One class of intrathymic IL-2 producers was found exclusively in the fetalthymus. Cells belonging to this subset were restricted to the outermost region of the thymus. IL-2expression in the fetal thymus was highly transient; a dramatic peak ofiL-2 mRNA accumulation was identified at day 14.5 of gestation and maximal IL-2 protein production was observed 12hours later, after which both IL-2 mRNA and protein levels rapidly decreased. Significantly, thepresence of IL-2 expressing cells in the day 14-15 fetal thymus was not contingent on thegeneration of T-cell receptor (TcR) positive cells. The second class of IL-2 producing cells wasalso detectable in the fetal thymus (cells found in this class represented a minority subset of IL-2producers in the fetal thymus) but persist in the thymus during later stages of development andafter birth. Intrathymic IL-2 producers in postnatal animals were located in the subcapsular regionand cortex, indicating that these cells reside in the same areas where immature T cells areconsigned. The frequency of IL-2 expressing cells in the postnatal thymus was extremely low,indicating that induction of IL-2 expression and protein synthesis are indicative of a rare activationevent. Unlike the fetal class of intrathymic IL-2 producers, the presence of IL-2 producing cells inthe postnatal thymus was dependent on to the generation of TcR+ cells. Subsequent examinationof intrathymic IL-2 production in mutant postnatal mice unable to produce either αβ or γδ T cellsshowed that postnatal IL-2 producers in the thymus belong to both αβ and γδ lineages.Additionally, further studies indicated that IL-2 synthesis by immature αβ -T cells depends on theexpression of bonafide TcR αβ-heterodimers. Taken altogether, IL-2 production in the postnatalthymus relies on the generation of αβ or γδ-TcR^+ cells and induction of IL-2 protein synthesis canbe linked to an activation event mediated via the TcR.
With regard to tissue specificity of IL-2 gene expression in vivo, analysis of whole bodysections obtained from normal neonatal mouse pups by in situ hybridization demonstrated that IL-2mRNA^+ cells were found in both lymphoid and nonlymphoid tissues with which T cells areassociated, such as the thymus (as described above), dermis and gut. Tissues devoid of IL-2mRNA^+ cells included brain, heart, lung, liver, stomach, spine, spinal cord, kidney, and bladder.Additional analysis of isolated tissues taken from older animals revealed that IL-2 expression wasundetectable in bone marrow and in nonactivated spleen and lymph nodes. Thus, it appears thatextrathymic IL-2 expressing cells in nonimmunologically challenged animals are relegated toparticular epidermal and epithelial tissues in which characterized subsets of T cells reside and thatinduction of IL-2 gene expression associated with these tissues may be a result of T-cell activation therein.
Based on the neonatal in situ hybridization results, a detailed investigation into possibleinduction of IL-2 expression resulting in IL-2 protein synthesis in the skin and gut revealed thatIL-2 expression is induced in the epidermis and intestine and IL-2 protein is available to drive cellproliferation of resident cells and/or participate in immune function in these tissues. Pertaining toIL-2 expression in the skin, maximal IL-2 mRNA accumulation and protein production wereobserved when resident Vγ_3^+ T-cell populations were expanding. At this age, both IL-2 mRNA^+cells and IL-2 protein production were intimately associated with hair follicles. Likewise, at thisage a significant number of CD3ε^+ cells were also found in association with follicles. Thecolocalization of IL-2 expression and CD3ε^+ cells suggests that IL-2 expression is induced when Tcells are in contact with hair follicles. In contrast, neither IL-2 mRNA nor IL-2 protein werereadily detected once T-cell density in the skin reached steady-state proportions. At this point, Tcells were no longer found associated with hair follicles but were evenly distributed throughout theepidermis. In addition, IL-2 expression in the skin was contingent upon the presence of mature Tcells therein and induction of IL-2 protein synthesis in the skin did not depend on the expression ofa specific TcR on resident T cells. These newly disclosed properties of IL-2 expression in the skinindicate that IL-2 may play an additional role in controlling mature T-cell proliferation byparticipating in the extrathymic expansion of T cells, particularly those associated with theepidermis.
Finally, regarding IL-2 expression and protein synthesis in the gut, IL-2 producing cellswere found associated with the lamina propria of neonatal animals and gut-associated IL-2production persisted throughout life. In older animals, the frequency of IL-2 producing cells in thesmall intestine was not identical to that in the large intestine and this difference may reflect regionalspecialization of the mucosal immune system in response to enteric antigen. Similar to otherinstances of IL-2 gene expression in vivo, a failure to generate mature T cells also led to anabrogation of IL-2 protein production in the gut. The presence of IL-2 producing cells in theneonatal gut suggested that these cells may be generated during fetal development. Examination ofthe fetal gut to determine the distribution of IL-2 producing cells therein indicated that there was atenfold increase in the number of gut-associated IL-2 producers at day 20 of gestation compared tothat observed four days earlier and there was little difference between the frequency of IL-2producing cells in prenatal versus neonatal gut. The origin of these fetally-derived IL-2 producingcells is unclear. Prior to the immigration of IL-2 inducible cells to the fetal gut and/or induction ofIL-2 expression therein, IL-2 protein was observed in the fetal liver and fetal omentum, as well asthe fetal thymus. Considering that induction of IL-2 protein synthesis may be an indication offuture functional capability, detection of IL-2 producing cells in the fetal liver and fetal omentumraises the possibility that IL-2 producing cells in the fetal gut may be extrathymic in origin and IL-2producing cells in these fetal tissues may not belong solely to the T lineage. Overall, these resultsprovide increased understanding of the nature of IL-2 producing cells in the gut and how theabsence of IL-2 production therein and in fetal hematopoietic tissues can result in the acutepathology observed in IL-2 deficient animals.
