A Monograph on Histomorphologic Evaluation of Lymphoid Organs

Shortly after the 2005 publication of the “Society of Toxicologic Pathology (STP) Position Paper on Best Practice Guidelines for the Routine Pathology Evaluation of the Immune System” (Haley et al., 2005), the National Toxicology Program (NTP) Satellite Symposium that preceded the 2005 annual STP meeting provided an interactive forum on this topic. Using specific examples of normal and abnormal lymphoid tissue histology, both during and after the NTP Satellite Symposium, it became apparent that a set of specific examples to illustrate the STP position paper would be useful to the toxicologic pathology scientific community. The present Monograph on the Histomorphologic Evaluation of Lymphoid Organs represents that endeavor. In the process of assembling the photomicrographs to support the STP Best Practices position paper, we realized that an illustrated review of normal structure, function, and histology of lymphoid organs as well as representative examples of common spontaneous and treatment-related lesions would provide useful background reference material.

This monograph consists of peer-reviewed papers covering normal structure, function, pathology, and enhanced histopathology for lymph nodes, thymus, bone marrow, spleen, and mucosa-associated lymphoid tissues. In addition there is a paper on the immunohistochemistry of lymphoid organs. Papers dealing with enhanced histopathology are grouped separately in this monograph. Hopefully, topics covered will serve to bring focus to some relevant issues such as the distinction between thymic atrophy and physiological involution and assessment of direct immunomodulatory effects versus effects secondary to stress. We have not covered methods related to collection, processing, sectioning, and staining of lymphoid tissues or recommended a uniform grading scheme for severity of tissue alterations.

The monograph emphasis is on rodent lesions with occasional addition of canine and nonhuman primate examples and is intended to be a guide and atlas for the general practicing toxicologic pathologist. All images are from hematoxylin and eosin-stained slides unless otherwise noted in the legend. The majority of images are produced in grayscale to maintain reasonable publication costs. All images are available in color on a CD-ROM inserted into this Monograph issue of Toxicologic Pathology. The images on the CD are of suitable resolution for teaching purposes.

The occasional redundancy between papers and between the text and legends, plus some duplication of photomicrographs is a deliberate attempt to have individual papers as well as the enclosed CD of images be sufficiently comprehensive to stand-alone as a definitive resource. There was also a deliberate decision not to include a lymphoma classification scheme or to favor a specific systemized nomenclature for lesions. Lymphoma classifications are extensively covered in the literature (Harleman and Jahn, 1990; Pattengale, 1990a, 1990b; Frith et al., 1996; Morse et al., 2002; Ward, 2006), and the STP is currently in the process of working with international colleagues to update a systemized nomenclature for lymphoid organs.

The STP best practice guidelines for the immune system calls for performing enhanced histopathology on lymphoid tissues consisting of descriptive and semiquantitative evaluation of the subcompartments of each lymphoid organ or tissue. Based on the title of the “Best Practice Guidelines” paper and specifically the inclusion of “routine” in the title, the implication is that this enhanced approach should be routinely applied. That seems neither appropriate nor practical. So, when does one do enhanced histopathology, or when is it appropriate not to do an enhanced histopathology evaluation of lymphoid tissues?

Enhanced histopathology would generally not be appropriate for use in chronic toxicity/carcinogenicity rodent studies. Its use on short-term (e.g., 28-day to 90-day) rodent toxicity studies is typically predicated on regulatory requirements or suspicion of a potential immunomodulatory effect. When used on specific studies, enhanced histopathology on lymphoid tissues is part of the total histopathological assessment of the study. Furthermore, it is not recommended that enhanced histopathology necessarily be done for all short-term studies. Rather, its use could be considered in the following situations:

• routine histopathology indicates that there may be an immunomodulatory effect,

• there is a reason to suspect an immunomodulatory effect based on information such as structural similarity to other immunotoxins or known physiological properties, or

• the end use of the agent under study is in an especially vulnerable population, in which case there would likely be existing regulatory guidelines for immunotoxicity testing.

This approach should alleviate the concerns that some have about enhanced histopathology of the immune system tissues being a mandatory part of the histological evaluation in all toxicity/carcinogenicity studies. Enhanced histopathology can be carried out either concurrently with, or retrospectively to, conventional histopathology that identifies “classic” lesions such as granulomas, abscesses, infarcts, metaplasia, pigmentation, etc. In a sense, this approach is similar to the extended evaluation of compartments of the liver (centrilobular, periportal), kidney (cortex, medulla, pelvis) or brain (cerebrum, brain stem, hippocampus, cerebellum) that we carry out when there is a specific need for more detailed data.

Furthermore, the enhanced histopathology is done on lymphoid organs and, when used alone, does not necessarily constitute a sufficiently adequate evaluation of the immune system in the absence of other endpoints such as body and organ weights, hematology, and clinical chemistry. The added value of enhanced histopathology derives from the opportunity for identification of immunotoxic potential by virtue of the semiquantitative evaluation of specific lymphoid tissues compartments. A truly comprehensive evaluation of the immune system requires additional studies as defined in various multitiered testing schemes (Luster et al., 1988, 1993; Van Loveren and Vos 1989; IPCS, 1996; ICICIS, 1998; Hinton, 2000).