Abstract
Melanomacrophage centers (MMCs) are aggregates of highly pigmented phagocytes found primarily in the head kidney and spleen, and occasionally the liver of many vertebrates. Preliminary histological analyses suggested that MMCs are structurally similar to the mammalian germinal center (GC), leading to the hypothesis that the MMC plays a role in the humoral adaptive immune response. For this reason, MMCs are frequently described in the literature as “primitive GCs” or the “evolutionary precursors” to the mammalian GC. However, we argue that this designation may be premature, having been pieced together from mainly descriptive studies in numerous distinct species. This review provides a comprehensive overview of the MMC literature, including a phylogenetic analysis of MMC distribution across vertebrate species. Here, we discuss the current understanding of the MMCs function in immunity and lingering questions. We suggest additional experiments needed to confirm that MMCs serve a GC-like role in fish immunity. Finally, we address the utility of the MMC as a broadly applicable histological indicator of the fish (as well as amphibian and reptilian) immune response in both laboratory and wild populations of both model and non-model vertebrates. We highlight the factors (sex, pollution exposure, stress, stocking density, etc.) that should be considered when using MMCs to study immunity in non-model vertebrates in wild populations.
Highlights
The study of immunology in wild vertebrates is hamstrung because many tools cannot readily be used in the field, and species-specific reagents do not exist for most non-model organisms
This review focuses on melanomacrophage center (MMC) immune functions but touches briefly on non-immunological roles [detailed review by Wolke [5]]
The MMC shares many structural, cellular, and molecular similarities with the mammalian germinal center (GC), suggesting an evolutionary tie to mammalian adaptive immunity. Parallels between these structures led researchers to recognize the potential for the MMC as a histological biomarker of poikilotherm immune response
Summary
The study of immunology in wild vertebrates is hamstrung because many tools cannot readily be used in the field, and species-specific reagents do not exist for most non-model organisms. It is possible that the MMC response may not be immunological like the GC but may be a non-specific expansion of MMs in response to generalized tissue damage In this case, the proposed immune function of MMCs may be a false lead. MMC finescale structure should be defined, for instance using single molecule fluorescent in situ hybridization to generate spatially explicit maps of gene expression and the distribution of cell types within MMCs. In the absence of functional studies that directly test the hypothesis of a GC-like MMC, an abundance of caution should be used when drawing conclusions regarding the nature of the MMC response in immune function
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