In the present study, we sought to determine whether the Th17/IL-23 system might be involved in the development of SS-like disease in C57BL/6.NOD-Aec1Aec2
mice and/or SS in humans. By taking advantage of the mouse model, it was possible to investigate whether there are temporal relationships between the Th17/IL-23 system, glandular inflammation, and clinical disease. Initial results indicate that both Th17 cells and IL-23–positive cells are present in the salivary glands, primarily located within the lymphocytic foci considered important and within prominent pathologic lesions associated with SS in both species. In contrast, and somewhat unexpectedly, lymphocytic foci occurring in older SS-nonsusceptible C57BL/6J mice (age >30 weeks) did not contain either Th17 cells or IL-23–positive cells, possibly indicating that these lymphocytic infiltrations are not pathologic lesions. Overall, these data therefore suggest, but do not prove, that these cell populations may play an important role in the development and/or onset of SS; however, there are no indications that either cell population is an actual effector of SS or that the levels of staining correspond to time of disease onset and/or severity. Nevertheless, the present study adds SS to the rapidly expanding list of autoimmune diseases (e.g., CD [10
], EAE [12
], multiple sclerosis [37
], rheumatoid arthritis [RA] [38
], psoriasis [39
], and CIA [13
]) in which the Th17/IL-23 system is now implicated.
While submandibular glands from diseased C57BL/6.NOD-Aec1Aec2
mice and lip salivary gland biopsy specimens from SS patients immunostained for IL-23 and IL-17 showed similar results, detection of these 2 cytokines in sera and saliva provided very different outcomes. In our mouse model, IL-17 could be detected only in sera prepared at early time points (i.e., up to age 16 weeks) when IL-17 could also be detected in sera from SS-nonsusceptible parental C57BL/6J mice, although in lesser quantities. In contrast, IL-17 was virtually undetectable in saliva at any of the time points examined (i.e., up to age 20 weeks). In the human specimens, IL-17 could be detected in 9 of the 21 serum samples from SS patients tested, a frequency similar to that in control subjects without SS (8 of 19). At the same time, IL-17 was detected in 11 of 21 saliva samples from SS patients, but in only 9 of 19 saliva samples from control subjects without SS. These data are consistent with those reported for RA patients, where IL-17 was not detected in serum samples but did appear in sera and synovial fluids from a subset of patients (33
While the results of our current study involving cytokines seem inconsistent, they may actually represent the predicted changing profile associated with the natural progression of the SS disease state. For example, based on data obtained from the human samples, we would predict that as the disease in the C57BL/6.NOD-Aec1Aec2 mice progresses beyond the 20 weeks followed up in the current study, IL-17 should eventually be detected in the saliva of these mice as well. Conversely, based on our data obtained from the mouse studies, the level of IL-17 would be anticipated to be elevated in the sera of SS patients if examined at a time point prior to clinical disease.
In the present study, we found that at age 4 weeks, C57BL/6.NOD-Aec1Aec2
mice exhibited up-regulation of T-bet,
the Th1 cell master control gene. This observation is consistent with previous studies by our group showing high levels of IFNγ
production at this early age (40
). Thus, our data are consistent with the concept that there is an early induction of a CD4+ Th1/Th17 pathway leading to systemic release of IL-17. Since IL-17 can induce an up-regulation of vascular cell adhesion molecule 1 that, in turn, makes vascular endothelium more adherent to intravascular lymphocytes, sites of inflammation would exhibit increased vascular adherence, permitting Th17 cells to gain access to the damaged tissues and to begin secreting cytokines that exacerbate the pathology (41
). This cascade can include IL-6, which induces expression of intercellular adhesion molecule 1, which functions as a receptor for activated T cells expressing lymphocyte function–associated antigen 1, as well as Th2 cytokines that function to induce infiltrating B lymphocytes to produce autoreactive antibodies (42
). Again, the coordinate up-regulation of Gata3
, the Th2 cell master control gene, and of RORγt
, the Th17 cell master control gene, is consistent with such a model.
Despite the apparent involvement of Th17 cells in the development and/or onset of SS, one cannot forget the importance of both the Th1 and Th2 pathways. In previous studies by our group, elimination of IFNγ
in SS-susceptible mice ameliorated all pathologic and clinical signs of disease, while elimination of IL-4 prevented loss of secretory function (40
). These observations indicate an essential role for both Th1 cell–and Th2 cell–associated cytokines. These data raise several important questions, including the question of when Th17 cells become involved in the autoimmune response and whether they act directly through secretion of inflammatory IL-17 family cytokines or by activating autoimmune T and B cells. If Th17 cells act in SS as reported in EAE, then IL-17 may not be required for initiation of SS, but it still represents a critical regulator of Th1 cells and their production of IFNγ
), again consistent with our group’s data from the study of Ifng
gene–knockout mice (40
). In any event, the IL-23/CD4+ Th17/IL-17R system appears to bridge aspects of the innate and adaptive immune responses.
Finally, is it possible to utilize these new data concerning the Th17/IL-23 system to actually identify developing SS-like or clinical SS disease? In both our C57BL/6.NOD-Aec1Aec2 mouse model and a randomly selected patient population, we observed various overlaps in data with presumably nonsusceptible or normal, healthy controls (e.g., the presence of IL-17 in saliva from humans or the presence of IL-17 in sera from mice). Furthermore, no particular disease profile appears to correlate specifically with our Th17/IL-23 findings, although the number of patients (and animals) included in the present study is limited. SS is a very intricate and complex autoimmune disease in which significant alterations, both physiologic and immunologic, are disease phase dependent or disease stage associated. Future investigations need to take these issues into consideration in designing studies to examine the systemic and localized effects of the Th17/IL-23 system in SS.