The development of lymphoid organs can be viewed as a continuum. At one end are the 'canonical' secondary lymphoid organs, including lymph nodes and spleen; at the other end are 'ectopic' or tertiary lymphoid organs, which are cellular accumulations arising during chronic inflammation by the process of lymphoid neogenesis. Secondary lymphoid organs are genetically 'preprogrammed' and 'prepatterned' during ontogeny, whereas tertiary lymphoid organs arise under environmental influences and are not restricted to specific developmental 'windows' or anatomic locations. Between these two boundaries are other types of lymphoid tissues that are less developmentally but more environmentally regulated, such as Peyer's patches, nasal-associated lymphoid tissue, bronchial-associated lymphoid tissue and inducible bronchial-associated lymphoid tissue. Their regulation, functions and potential effects are discussed here.

To trigger an effective immune response, antigen and antigen-presenting cells travel to the lymph nodes via collecting lymphatic vessels. However, our understanding of the regulation of collecting lymphatic vessel function and lymph transport is limited. To dissect the molecular control of lymphatic function, we developed a unique mouse model that allows intravital imaging of autonomous lymphatic vessel contraction. Using this method, we demonstrated that endothelial nitric oxide synthase (eNOS) in lymphatic endothelial cells is required for robust lymphatic contractions under physiological conditions. By contrast, under inflammatory conditions, inducible NOS (iNOS)-expressing CD11b + Gr-1 + cells attenuate lymphatic contraction. This inhibition of lymphatic contraction was associated with a reduction in the response to antigen in a model of immune-induced multiple sclerosis. These results suggest the suppression of lymphatic function by the CD11b + Gr-1 + cells as a potential mechanism of self-protection from autoreactive responses during on-going inflammation. The central role for nitric oxide also suggests that other diseases such as cancer and infection may also mediate lymphatic contraction and thus immune response. Our unique method allows the study of lymphatic function and its molecular regulation during inflammation, lymphedema, and lymphatic metastasis.

The mature phenotype of peripheral lymph node (LN) high endothelial venules (HEVs), defined as MAdCAM-1lowPNAdhighLTβRhigh HEC-6SThigh, is dependent on signaling through the lymphotoxin-β receptor (LTβR). Plasticity of PLN HEVs during immunization with oxazolone was apparent as a reversion to an immature phenotype (MAdCAM-1highPNAdlowLTβRlow HEC-6STlow) followed by recovery to the mature phenotype. The recovery was dependent on B cells and was inhibited by LTβR-Ig treatment. Concurrent with HEV reversion, at day 4 following oxazolone or OVA immunization, reduced accumulation of Evans blue dye and newly activated DCs in the draining LNs revealed a temporary afferent lymphatic vessel (LV) functional insufficiency. T cell priming to a second Ag was temporarily inhibited. At day 7, lymphangiogenesis peaked in both the skin and draining LN, and afferent LV function was restored at the same time as HEV phenotype recovery. This process was delayed in the absence of B cells. LV and HEV both express the LTβR. During lymphangiogenesis in the draining LN, HEV, and LV were directly apposed; some vessels appeared to express both PNAd and LYVE-1. Pretreatment with LTβR-Ig drastically reduced the number of PNAd+LYVE-1+ vessels, suggesting a reduction in LV and HEV cross-talk. The concordance in time and function and the close physical contact between LVs and HEVs in the remodeling process after immunization indicate that the two vascular systems are in synchrony and engage in cross-talk through B cells and LTβR.

Although the role of TGF-β in tumor progression has been studied extensively, its impact on drug delivery in tumors remains far from understood. In this study, we examined the effect of TGF-β blockade on the delivery and efficacy of conventional therapeutics and nanotherapeutics in orthotopic mammary carcinoma mouse models. We used both genetic (overexpression of sTβRII, a soluble TGF-β type II receptor) and pharmacologic (1D11, a TGF-β neutralizing antibody) approaches to block TGF-β signaling. In two orthotopic mammary carcinoma models (human MDA-MB-231 and murine 4T1 cell lines), TGF-β blockade significantly decreased tumor growth and metastasis. TGF-β blockade also increased the recruitment and incorporation of perivascular cells into tumor blood vessels and increased the fraction of perfused vessels. Moreover, TGF-β blockade normalized the tumor interstitial matrix by decreasing collagen I content. As a result of this vessel and interstitial matrix normalization, TGF-β blockade improved the intratumoral penetration of both a low-molecular-weight conventional chemotherapeutic drug and a nanotherapeutic agent, leading to better control of tumor growth.breast cancer | vessel normalization | drug delivery B reast cancer is the second leading cause of cancer death in women, with most fatalities resulting from a failure to control metastatic disease with systemically administered therapies. In addition to the induction of cellular resistance mechanisms (decreased apoptosis, increased drug efflux, etc.), impaired intratumoral drug delivery is an important physiological factor contributing toward chemoresistance (1, 2). TGF-β is an important regulator of normal mammary gland development and function, as well as of the progression of mammary carcinomas (3-8). Although the role of TGF-β in tumor progression and metastasis has been studied extensively, little is known about its impact on drug delivery.Transport of a therapeutic agent from the circulation to cancer cells is a three-step process. Systemically administered drugs must (i) travel to different regions within a tumor via the vascular network; (ii) cross the vessel wall; and finally (iii) diffuse through the interstitial space to reach the tumor cells, with each step being hindered by the presence of an abnormal vasculature and/or matrix (1, 2, 9, 10). Tumor blood vessels are structurally and functionally abnormal, characterized by increased permeability and heterogeneous perfusion. Poor vascular perfusion decreases drug delivery and, as a result, impairs the efficacy of blood-borne antitumor agents (1, 11). In addition, the dense collagen-rich interstitial matrix further hinders drug transport to tumor cells-a feature especially relevant to larger therapeutics, such as nanoparticles (1-100 nm) (1, 10, 12, 13). The dense collagen matrix also contributes to solid stress, which compresses tumor vessels (14). Hence, depleting collagen will reduce stress and open up compressed vessels. TGF-β is a negative regulator of pericyte recruitment during blood vessel stab...