The development of PP during late embryogenesis has been well characterized in recent years. This varies from development of other secondary lymphoid organs in some key ways. In lymph node organogenesis, early lymph sacs bud from vascular endothelium in a prospero-related homeobox 1 (PROX1)-dependent manner to accumulate LTi cells, while PP are initiated by the Ret
proto-oncogene-dependent clustering of previously scattered LTi cells with KIT+
, ITGAX (CD11C)+
initiator cells in the small intestine 
. The local microenvironments for these tissues require different cytokines and signaling pathways to induce lymphoid development. Whereas PP require JAK3-dependent IL7R signaling, lymph nodes require TRAF6-dependent TNFSF11 (TRANCE) signaling for the differentiation and function of LTi cells 
. Additionally, the signaling requirements for cervical and mesenteric lymph nodes can vary from those of other lymph nodes (reviewed in 
). Development of splenic white pulp is not dependent on LTi cells, but its organization and maturation requires activation of NFKB through LTA1B2 signaling in lymphocytes 
. Similarly, although pathways involving TNF, TNFRSF1A, LTA, LTB, TNFSF14 (LIGHT), and the LTBR all have complex roles in the development and organization of secondary lymphoid organs 
, LTA1B2 activation of NFKB transcription factors via the LTBR is indispensible for lymph node and PP development. The LTBR is one of several tumor necrosis factor (TNF) superfamily receptors, the binding of which lead to activation of inhibitor of kappa B kinases (IKKs) and MAP3K14 (NFKB inducing kinase [NIK]); these phosphorylate inhibitory proteins that sequester NFKB homo- and heterodimers in the cytoplasm, and subsequent ubiquitin-proteasome pathway degradation of the inhibitors allows nuclear translocation of NFKB complexes for transcription of target genes (reviewed in 
). Among genes downstream of the LTBR and other TNF superfamily receptors are those involved in inflammation, cell survival, and innate immunity (primarily through canonical activation of the NFKB1-P50 heterodimer), lymphoid tissue development (primarily through noncanonical activation of the NFKB2-P52 heterodimer), and organization of these structures (involving both pathways).
PP are absent in mice lacking Lta
, or the Ret
. Significant blunting of PP development is seen in mice lacking Cxcl13
or its receptor Cxcr5
and in some mice lacking Tnf
. It is interesting to note that in all of these mice, with the exception of those with defects in the IL7 receptor pathway, lymph node genesis is also affected. In contrast, adult Sharpincpdm
mutant mice have a full complement of lymph nodes, although these are often infiltrated by granulocytes, whereas they lack PP 
SHARPIN was recently discovered to be component of the RBCK1 (HOIL1L) and RNF31 (HOIP)-containing linear ubiquitin chain assembly complex. Linear ubiquitination of the IKK regulatory subunit IKBKG (NEMO) is among the functions of this complex, and SHARPIN deficiency was shown to result in impaired CD40- and TNFA-induced activation of the IKK complex and subsequent canonical NFKB activation in B cells and mouse embryo fibroblasts in vitro
. Although activation of the noncanonical NFKB pathway, which is essential for secondary lymphoid organ development, was not reported to be affected in B cells or mouse embryo fibroblasts, SHARPIN may potentially influence this pathway in other cell types or in specialized microenvironments during lymphoid tissue development. Investigators demonstrated that ablation of TNF in Sharpincpdm
mutant mice rescued the inflammatory skin phenotype and eliminated granulocyte accumulations in various organs, but did not rescue development of the splenic marginal zone or PP 
. Thus, the inflammatory cell infiltrates observed in regressing PP of young mice in the present study are not likely to be the cause of PP loss. Dysregulation of local chemokine expression in high endothelial venules and stromal tissues may contribute to the accumulation of granulocytes in secondary lymphoid organs in this disease.
Findings of the present study demonstrate that while PP development is initiated in Sharpincpdm
mutant mice, essential compartmentalization which normally begins prior to the entry of lymphocytes fails. Efficient segregation into B and T cells areas is lacking, and the previously described absence of FDCs, possibly a consequence of the faulty B cell maturation or defects in stromal FDC precursors, leads to absence of B cell follicles 
. Changes in the spleen, although less severe, parallel those observed in the PPs. Increased extramedullary myelopoiesis accompanies accumulation of granulocytes in the spleens of mutant mice over time. Whereas organization, compartmentalization, and lymphocyte accumulations increase with maturity in spleens of controls, a more primitive white pulp structure is evident in mutant mice, with increased mixing of B and T cells beginning between 2 and 4 weeks of age, and failure of these compartments to expand.
Cooperative signaling between stromal tissues and hematopoetic cells regulates cell survival, differentiation, and proliferation, as well as the segregation and trafficking of B and T lymphocytes in secondary lymphoid organs during development and in immune responses. Loss of one or more of these signals likely leads to the lack of immunologically competent compartments and eventual regression of PP in Sharpincpdm mutant mice.
is conserved among several species, including mice and humans 
. Such evolutionary conservation suggests a biological advantage. Results of this and other studies suggest that SHARPIN has essential roles in secondary lymphoid organ development, maturation of B cells and subsequent immunoglobulin production, and control of inflammation. At least some of the developmental abnormalities observed in Sharpincpdm
mutant mice in the present study appear to be caused by the absence of SHARPIN in stromal cells, because the compartmentalization of PP stromal cells is independent of lymphocyte entry 
. Furthermore, irradiation of juvenile Sharpincpdm
mutant mice and transplantation with bone marrow from healthy donors at a time when PP are present did not prevent complete regression of PP by 8 weeks of age.
Despite a lack of normal organization, lymphocytes were recruited into PP anlagen in neonatal mutant mice, indicating that SHARPIN is essential for the organization and maintenance, but not the initial development of PP.