Although every step is essential for the reproducibility of this colitis model, there are several critical steps. The recipient Rag-/- mice should receive adequate viable naϊve CD4+ T cells to induce intestinal inflammation. We used spleens for the isolation of naïve CD4+ T cells instead of MLNs. Because the yield of naïve CD4+ T cells in MLNs is much lower than in spleens. CD62L is highly expressed in naïve T cells, and CD44 and CD25 are the activation markers of T cells13,14. In this study, we first used anti-CD4 magnetic beads to isolate CD4+ T cells from spleens. Then we used the combination of anti-CD4, anti-CD62L, and anti-CD25 antibodies for isolation of naïve CD4+ T cells13,14. Researchers could use other markers for sorting the naïve CD4+ T cells. CD45RBhi is also marker of naïve T cells. CD45RBhi CD25- CD4+ T cells are commonly used as naïve T cells of wild-type non-TCR transgenic T cells23. Therefore, the techniques of sorting cells and intravenously injection of the T cells into the recipient mice are essential. Setting up the gating protocol as a template is helpful to speed up the experiments with fewer errors. To avoid cell death, cells should always be kept on ice. Besides, staining cells with DAPI is highly recommended for excluding dead cells because DAPI cannot transit across intact cell membranes, making it an excellent dead cell probe24. Warming the mice to stimulate dilation of the tail veins provides better vein visibility for intravenous injection. All procedures are recommended to be performed by trained researchers.
Many factors might impact the outcome of the colitis models, which needs to be paid attention. First, the recipient Rag1-/- mice should be age and gender-matched. In the CD45RBhi T cell transfer model, T cells from male and female donors can be transferred to male Rag-/- recipients, while only female donors can be used when using female recipients23. However, we do not see a significant difference between male and female recipients. The recipient mice could be either females or males, and T cells from male donors can also induce colitis in female recipients. Since weight change is a valuable indicator of colitis progression, it is recommended to use the recipient mice between 8-12 weeks to present a stable weight line. These recipient mice should be bred and kept in the same room of the animal facility because microbiota is critical in regulating colitis development25. The time to develop colitis varies when transferring different numbers of CBir1 TCR Tg naïve CD4 T cells. As expected, fewer cells require a longer time for induction of colitis, and a higher number of cells require a shorter time for induction of colitis. Using 1 x 106 cells per recipient mouse is recommended since the recipient mice demonstrate clinical signs of colitis ~2-3 weeks post cell transfer and develop relatively severe colitis ~6 weeks post cell transfer. In addition, compared with intraperitoneal injection, intravenous injection of cells into the tail vein induces more consistent colitis. For isolation of colonic lamina propria cells, the colon tissues must not get dry; otherwise, it would reduce cells' yield and viability. One of the particular concerns is that the duration of the colitis would be changed if the recipient mice are transferred with genetic-modified CBir1 TCR Tg T cells or treated with drugs26. In addition, CBir1 TCR Tg T cells also induce intestinal inflammation in other immune-deficient mice, such as TCRβ/δ-/- mice, which lack T cells27.
As accumulating evidence indicates a crucial role of gut bacterial antigen-specific reactive T cells in the pathogenesis of IBD, using T cells specific for a defined gut bacterial antigen will provide insights into how gut bacterial antigen induce T cell responses to induce colitis. Gut microbiota antigen CBir1 flagellin is abundant in the gastrointestinal tract, which is related to the pathogenesis of IBD8,9. This colitis model resembles several critical characteristics of IBD, including diarrhea, weight loss, histopathological finding, and abnormal intestinal immune responses. Therefore, this colitis model is useful to study the mechanisms of human IBD and provides a tool to evaluate the treatments for IBD. Interestingly, the recent work of Chiaranunt et al. indicated that T cell specificity to the microbiota CBir 1 antigen alone might not be sufficient to induce T cell activation and colitis. This is evidenced by the wild-type CBir1 TCR Tg T cells induced colitis in Rag-/- recipients, whereas Rag-/- CBir1 T cells did not induce colitis in their animal facility, suggesting that gut T cells responding to specific gut bacterial antigen may require other interrelated commensal bacteria, for example, Helicobacter spp, to function as an adjuvant28. An exciting aspect of this model is that different T cell subsets, namely Th1, Th17, and Treg cells, are present in lamina propria of colitic recipient mice, which provides a unique opportunity for investigating the roles of not only effector T cells but also Tregs in the pathogenesis of colitis29.
However, as this colitis model is mediated by gut microbiota-specific T cells, one limitation for this colitis model is that the duration of induction of colitis may vary in different animal facilities depending on gut microbiota.