Characterization of lymphoid tissue inducer cells and lymphoid tissue development in adult interleukin 7 transgenic mice

During embryogenesis, the development of secondary lymphoid organs (SLOs)
such as lymph nodes (LNs) and Peyer’s patches (PPs) requires the cellular crosstalk
between vascular cell adhesion molecule (VCAM)-1+ mesenchymal organizer cells
and CD45+CD4+lin- lymphoid tissue inducer (LTi) cells. The cascade of events
leading to functional SLOs is triggered by the activation of the lymphotoxin β receptor
(LTβR) signalling pathway. LTi cells express the corresponding ligand lymphotoxin
(LT) αβ, and other tumor necrosis factor super-family members, chemokine
receptors, adhesion molecules and Interleukin 7 Receptor alpha (IL-7Rα), which
contribute to the formation of SLOs. However, the precise mechanism of surface
receptor engagement for lympho-organogenesis and LTi cell function was not fully
understood. In addition, it remained unclear if LTi cells could persist in adult mice,
and had a function in the adult immune system.
In order to better understand the role of IL-7 in SLO development, we generated a
double transgenic mouse model overexpressing IL-7 under the control of an
ubiquitous promoter (termed H-IL-7). These mice developed additional ectopic LNs
and PPs (1). Ectopic SLO development was strictly dependent on LTi cells. We
further showed that the development of ectopic SLOs was mediated by an IL-7-
driven increase in the survival of fetal LTi cells and its progenitors.
CD4+lin- cells were found in significant numbers in all SLOs of adult H-IL-7 mice.
This study was performed to characterize CD4+lin- cells in adult mice, and to identify
their function. Adult CD4+lin- cells shared the phenotype with fetal LTi cells, including
the expression of retinoic acid-related orphan receptor (ROR) γt. By transferring adult
CD4+lin- cells into PP-deficient CXCR5-/- mice, we demonstrated their ability to
generate lymphoid tissue. Thus, adult CD4+lin- cells were bona fide LTi cells.
In order to test if adult LTi cells were present in normal wild type (WT) mice, and
could respond to IL-7, we treated adult WT mice with IL-7/anti-IL-7 antibody (Ab)
complexes. The pool of LTi cells was significantly increased in treated as compared
to untreated mice demonstrating that adult LTi cells were responsive to IL-7.
We further investigated the origin of adult LTi cells. We could show that the adult
bone marrow (BM) could give rise to LTi cells, which was even more pronounced,
when normal WT mice were treated with IL-7/anti-IL-7 Ab complex. BM cells were,
however, far less efficient in generating LTi cells than fetal liver (FL) cells.
It is well established that chronic inflammatory diseases in humans are often
associated with a process termed "lymphoid neogenesis". Lymphoid neogenesis
leads to the development of tertiary lymphoid organs (TLOs) in non-lymphoid organs.
In several autoimmune diseases, a correlation between TLO development and IL-7
production has been reported, but experimental evidence for a causal role of IL-7 in
TLO development was lacking. In adult H-IL-7 mice, we observed the development of
TLOs in several non-lymphoid organs such as the salivary gland. Moreover, these
TLOs were either diffuse or segregated in B and T cell areas, a hallmark of normal
SLOs. LTi cells colonized the salivary gland before naive lymphocytes, but were not
strictly required for TLO development. In contrast, the expression of LTα was
essential for the organization of TLOs into segregated compartments.
To test if local inflammation could trigger the development of TLOs in H-IL-7 mice,
we infected mice s.c. with Leishmania major. At sites of infection but not in noninfected
H-IL-7 mice, we found additional ectopic LNs. Moreover, the number of LTi
cells was significantly increased in the draining LNs of both WT and H-IL-7 infected
mice compared to non-infected. Altogether, these data show that the overexpression
of IL-7 was able to induce lymphoid neogenesis at ecopic sites.
In summary, this study shows that IL-7 is an important cytokine for the regulation
of TLO development in adult mice. We have identified adult LTi cells, which infiltrate
TLOs and may have a function in organizing the architecture of TLOs through
activation of the LTβR signalling pathway.