Characterization of Pipefish Immune Cell Populations Through Single-Cell Transcriptomics.

Jamie Parker,Naomi Croft Guslund,Sissel Jentoft,Olivia Roth

doi:10.3389/fimmu.2022.820152

Abstract

Teleost adaptive immune systems have evolved with more flexibility than previously assumed. A particularly enigmatic system to address immune system modifications in the evolutionary past is represented by the Syngnathids, the family of pipefishes, seahorses and seadragons. These small fishes with their unique male pregnancy have lost the spleen as an important immune organ as well as a functional major histocompatibility class II (MHC II) pathway. How these evolutionary changes have impacted immune cell population dynamics have up to this point remained unexplored. Here, we present the first immune cell repertoire characterization of a syngnathid fish (Syngnathus typhle) using single-cell transcriptomics. Gene expression profiles of individual cells extracted from blood and head-kidney clustered in twelve putative cell populations with eight belonging to those with immune function. Upregulated cell marker genes identified in humans and teleosts were used to define cell clusters. While the suggested loss of CD4+ T-cells accompanied the loss of the MHC II pathway was supported, the upregulation of specific subtype markers within the T-cell cluster indicates subpopulations of regulatory T-cells (il2rb) and cytotoxic T-cells (gzma). Utilizing single-cell RNA sequencing this report is the first to characterize immune cell populations in syngnathids and provides a valuable foundation for future cellular classification and experimental work within the lineage.

Highlights

The vertebrate immune system has evolved into an extremely diverse, layered network of defense mechanisms capable of combatting a wide variety of agents on a specific and generic level [1]
From three S. typhle pipefish individuals a total of 12129 genes across 8658 cells were accrued following filtering and quality control measures. 2196 cells were derived from blood and 6462 were derived from the head kidney
The putative function and classification for cells belonging to the largest cluster central to the Uniform Manifold approximation and projection (UMAP) visualization, was hematopoietic or progenitor immune cells (4,508 cells)

Summary

Introduction

The vertebrate immune system has evolved into an extremely diverse, layered network of defense mechanisms capable of combatting a wide variety of agents on a specific (adaptive) and generic (innate) level [1]. Pipefish Immune Cell Population Characterization and Hippocampus missing a functional Major Histocompatibility Complex II (MHC II) pathway [4, 11] Until recently this pathway was thought to be synonymous across vertebrates, it has been postulated that the evolution of full pregnancy with placenta-like structures in syngnathids may have been facilitated by this adaptive immune rearrangement [4]. The presence of a fully functional MHC II repertoire in other pipefish species (e.g., Nerophinae), with basal male pregnancy without placentalike structures, presents the lineage as an intriguing subject for immune and evolutionary studies. These immunological adaptations within the group were suggested to have coevolved with male pregnancy permitting immunological tolerance towards the semi-allogenic embryo [4, 12]. Whilst recent studies have helped elucidate the syngnathid immune networks and processes, the evolutionary repercussions of such immune deficiencies in the immune cell populations remain unknown, with no established immune cell characterization of any syngnathid species

Methods

Results

Conclusion