Abstract

Introduction

   Peyer's patches (PPs) are crucial antigen-inductive sites of intestinal
   mucosal immunity. Prior research indicated that, in contrast to other
   ruminants, PPs in the small intestine of Bactrian camels are found in
   the duodenum, jejunum, and ileum and display polymorphism. Using this
   information, we analyzed the microbial and metabolic characteristics in
   various segments of the Bactrian camel's small intestine to further
   elucidate how the immune system varies across different regions.

Methods

   In this study, the microbiota and metabolite of 36 intestinal mucosal
   samples, including duodenal (D-PPs), jejunal (J-PPs), and ileal PPs
   (I-PPs), were profiled for six Bactrian camels using 16S rRNA gene
   sequencing and liquid chromatography with tandem mass spectrometry
   (LC-MS/MS). To confirm meaningful associations, we conducted connection
   analyses on the significantly different objects identified in each
   group's results. ELISA was used to analyze the levels of IgA, IgG, and
   IgM in the same tissues.

Results

   The microbiota and metabolite profiles of J-PPs and I-PPs were found to
   be similar, whereas those of D-PPs were more distinct. In J-PPs and
   I-PPs, the dominant bacterial genera included Clostridium,
   Turicibacter, and Shigella. In contrast, D-PPs had a significant
   increase in the abundance of Prevotella, Fibrobacter, and
   Succinobacter. Regarding the metabolomics, D-PPs exhibited high levels
   of polypeptides, acetylcholine, and histamine. On the other hand, J-PPs
   and I-PPs were characterized by an enrichment of free amino acids, such
   as L-arginine, L-glutamic acid, and L-serine. These metabolic
   differences mainly involve amino acid production and metabolic
   processes. Furthermore, the distribution of intestinal immunoglobulins
   highlighted the specificity of D-PPs. Our results indicated that
   proinflammatory microbes and metabolites were significantly enriched in
   D-PPs. In contrast, J-PPs and I-PPs contained substances that more
   effectively enhance immune responses, as evidenced by the differential
   distribution of IgA, IgG, and IgM.

Discussion

   The intestinal microenvironment of Bactrian camels displays distinct
   regional disparities, which we propose are associated with variations
   in immunological function throughout different segments of the small
   intestine. This study highlights the specific traits of the intestinal
   microbiota and metabolites in Bactrian camels, offering a valuable
   reference for understanding the relationship between regional
   intestinal immunity and the general health and disease of the host.

   Keywords: Bactrian camels, intestinal microbiota, metabolites, Peyer’s
   patches, intestinal regional immunity

1. Introduction

   The gut, housing a diverse array of immune cells with unique
   properties, is the largest immunological organ in both humans and
   animals. The gastrointestinal system relies on several immune
   compartments to effectively respond to the numerous antigens and
   stimuli it encounters ([39]1). The mesenteric lymph nodes and
   gut-associated lymphoid tissue, as induction sites, play a crucial role
   in initiating immune responses by capturing, processing, and presenting
   antigens. Peyer’s patches (PPs) serve as a prime example of this
   function. Intestinal lamina propria and epithelial cells serve as
   effector sites, where different types of immune cells are responsible
   for clearing antigens to maintain the integrity of the barrier.
   Trillions of bacteria reside in the gut and are essential for enhancing
   nutrient absorption and energy metabolism ([40]2, [41]3), maintaining a
   healthy immune system ([42]4), and preventing or treating intestinal
   diseases ([43]5). The intestinal mucosal immune system safeguards the
   body against pathogens and ensures the ability to tolerate commensal
   microbes through the utilization of specialized recognition systems for
   both innate and adaptive immune responses. Recent research has
   emphasized the segmental distribution characteristics of the immune
   system in the intestines, such as lymph nodes, dendritic cells,
   intestinal epithelial cells, T helper 17 cells (Th17) and regulatory T
   cells (Treg), and antibody production ([44]6–[45]9). The placement of
   these components in different sections of the gut is strategic in order
   to meet the specific immunological needs of each area. Both immune
   cells and symbiotic microorganisms in the body exhibit similar regional
   distribution patterns. The variations of the physical and chemical
   environment throughout different sections of the gut leads to different
   composition of bacteria ([46]10). The small intestine harbors a more
   extensive variety of microbial diversity compared to the distal
   intestine ([47]11). Moreover, the microbial populations of the small
   intestine can be distinguished in the duodenum, jejunum, and ileum
   ([48]12–[49]14). The gut microbiota has a direct impact on
   gastrointestinal function and controls gastrointestinal physiology by
   producing various metabolites ([50]15). Metabolites such as amino
   acids, polyamine compounds, short chain fatty acids, and aromatic
   metabolites are play a role in regulating the local immunity of the
   intestine ([51]16). Studies suggest that microbial activity plays a
   significant role in shaping the metabolic status of various ecological
   niches across the gastrointestinal system, leading to the development
   of unique metabolic profiles in different parts ([52]17). Furthermore,
   alterations in gut microbiota have been associated with a range of
   immunological diseases, such as inflammatory bowel disease ([53]18),
   diabetes ([54]19), and atopic disorders ([55]20). Exploring the
   regional characteristics of gut microbiota can enhances our
   understanding of the links between host physiological processes and
   disease development.

   Bactrian camels (Camelus bactrianus) are large even-toed ungulates
   primarily found in China and other Central Asian regions. It has two
   distinct subspecies, including the domestic Bactrian camel (Camelus
   bactrianus) and the wild Bactrian camel (Camelus ferus). They can well
   adapt to the tough living conditions of deserts and cold locations.
   Camels exhibit greater resilience to specific viral illnesses and
   environmental stresses compared to other species inhabiting the same
   geographical area, owing to their distinctive immune traits ([56]21).
   Our earlier research showed that camels have a unique area of lymph
   aggregation in the abomasum, which is potentially associated with its
   distinct immunological attributes Although, this area has similar
   protein expression profile with ileal Peyer’s patches ([57]22), there
   are variations in the microbial communities ([58]23). Intestinal
   location affects the expression pattern of PPs genes, with
   significantly lower expression in the duodenum compared to the jejunum
   and ileum ([59]24). Consequently, the distribution of PPs in the
   duodenum is infrequently documented in both humans and animals.
   However, our study showed that PPs are present in all sections of the
   Bactrian camel’s small intestine, with variations in morphology and
   distribution density. In the duodenum, jejunum, and ileum, PPs exhibit
   honeycomb, nodular, and saccular structures, respectively ([60]25).
   This diversity may indicate regional differences in immune function
   within the small intestine. In PPs, a considerable proportion of
   lymphocytes undergo differentiation and activation, playing a pivotal
   role in both regulating the proliferation of symbiotic bacteria
   ([61]26) and producing a range of antibodies ([62]27). These antibodies
   effectively neutralize numerous antigens, thereby establishing a
   secondary defense line within the intestinal immune system. In
   addition, the gut microbiota and its metabolites exert intricate
   influences on adaptive immunity, such as regulating the production of
   IgA, IgM, and IgG ([63]23). These findings suggest that the
   distribution patterns of PPs in the gut of the Bactrian camel may be
   related to changes in regional immunological functions. The
   aforementioned research indicates the possibility of investigating the
   variations in the immune response of Bactrian camels’ intestinal
   mucosal inductive sites.

   To accomplish this objective, this study examined the relationships
   between the intestinal commensal microbiota, metabolites, and the host.
   Specifically, we obtained and compared the microbiota and metabolic
   profiles of the PPs in duodenal (D-PPs), jejunal (J-PPs), and ileum
   (I-PPs). On the other hand, the expression of IgA, IgG, and IgM was
   determined using an enzyme-linked immunosorbent assay (ELISA) for a
   mucosal immune characterization. It is expected to further comprehend
   the feature of small intestine segmental mucosal immunity in Bactrian
   camels and investigate the correlation between colonizing bacteria and
   metabolites. Moreover, the study can provide valuable references for