The Atlantic cod immune system deviates from antigen presentation processes seen in other vertebrates in that it lacks the necessary genes for exogenous antigen presentation (i.e., MHC-II and li) and a key MHC-II interacting molecule necessary for T-helper cell function (i.e., CD4), while possessing an expanded repertoire of MHC-I genes that facilitate endogenous antigen presentation. These observations, combined with the identification of putative endosomal sorting signals in MHC-I cytoplasmic tails, have led to speculation that cod rely on cross-presentation of exogenous antigens to elicit cytotoxic T-lymphocyte responses against extracellular threats. In light of this suggestion, we investigated MHC-I transcriptional profiles and endosomal sorting signals in a closely related gadoid species, the haddock. Analysis of transcripts from one individual identified 13 unique MHC-I molecules, including two non-classical molecules as determined by the level of conservation at their peptide anchoring sites. This suggests that like the cod, the haddock has an expanded MHC-I repertoire. Analysis of haddock MHC-I cytoplasmic tail sequences revealed that the dileucine- and tyrosine-based endosomal signaling motifs, that are suggested to facilitate cross-presentation in cod, were absent. Closer examination of the cod signaling motifs, including their relative position in the cytoplasmic tail region, indicates that these motifs might be non-functional, further supporting the need for functional studies to assess cross-presentation. Finally, in silico analysis and in vitro N-type de-glycosylation experiments demonstrate that haddock and cod beta-2-microglobulin (β2M) are glycosylated at the same NQT sequon. Interestingly, whole genome tBLASTn searches also revealed that putative β2 M glycosylation sites appear frequently within the Gadiformes lineage, as the predictive NQT and other N-X-S/T sequons were located in β2M orthologues from 19 of the 25 additional gadoid genomes analyzed. Though the exact significance of β2M glycosylation has yet to be elucidated, phylogenetic comparisons predict that the same NQT glycosylation sequence occurs in 13 additional species comprising four different orders of Actinopterygii (Gymnotiformes, Esociformes, Beryciformes and Perciformes). This suggests either that this feature has arisen independently in multiple lineages or that it comes from a common ancestor and has been lost or modified in many species. Together, the analysis of gadoid MHC-I genes and β2M molecules highlights the challenges in generalizing immune system paradigms across the most diverse vertebrate lineage (i.e., fish) and between fish and more well-studied mammals.

大西洋鳕鱼免疫系统与其他脊椎动物中的抗原呈递过程有所不同，因为它缺乏外源抗原呈递的必要基因（即MHC-II和li）以及T辅助细胞功能所必需的关键MHC-II相互作用分子（ （例如CD4），同时拥有有助于内源抗原呈递的MHC-1基因扩展库。这些观察结果与在MHC-1细胞质尾部中推测的内体分类信号的鉴定相结合，导致推测鳕鱼依赖于外源抗原的交叉呈递来引发针对细胞外威胁的细胞毒性T淋巴细胞反应。根据这一建议，我们研究了密切相关的类腺物种黑线鳕中的MHC-1转录概况和内体分类信号。从一个人进行的转录本分析确定了13个独特的MHC-1分子，其中包括两个非经典分子，这取决于其肽锚定位点的保守程度。这表明象鳕鱼一样，黑线鳕具有扩展的MHC-I库。对黑线鳕MHC-1细胞质尾序列的分析表明，缺乏基于双亮氨酸和酪氨酸的内体信号转导基序，这些基序被认为有助于在鳕鱼中进行交叉表达。对鳕鱼信号转导基序，包括它们在细胞质尾部区域中的相对位置，进行更仔细的检查，表明这些基序可能是非功能性的，从而进一步支持了功能性研究以评估交叉展示的需要。最后，包括两个非经典分子，这取决于其肽锚定位点的保守程度。这表明象鳕鱼一样，黑线鳕具有扩展的MHC-I库。对黑线鳕MHC-1细胞质尾序列的分析表明，缺乏基于双亮氨酸和酪氨酸的内体信号转导基序，这些基序被认为有助于在鳕鱼中进行交叉表达。对鳕鱼信号转导基序，包括它们在细胞质尾部区域中的相对位置，进行更仔细的检查，表明这些基序可能是非功能性的，从而进一步支持了功能性研究以评估交叉展示的需要。最后，包括两个非经典分子，这取决于其肽锚定位点的保守程度。这表明象鳕鱼一样，黑线鳕具有扩展的MHC-I库。对黑线鳕MHC-1细胞质尾序列的分析表明，缺乏基于双亮氨酸和酪氨酸的内体信号转导基序，这些基序被认为有助于在鳕鱼中进行交叉表达。对鳕鱼信号转导基序，包括它们在细胞质尾部区域中的相对位置，进行更仔细的检查，表明这些基序可能是非功能性的，从而进一步支持了功能性研究以评估交叉展示的需要。最后，对黑线鳕MHC-1细胞质尾序列的分析表明，缺乏基于双亮氨酸和酪氨酸的内体信号转导基序，这些基序被认为有助于在鳕鱼中进行交叉表达。对鳕鱼信号转导基序，包括它们在细胞质尾部区域中的相对位置，进行更仔细的检查，表明这些基序可能是非功能性的，从而进一步支持了功能性研究以评估交叉展示的需要。最后，对黑线鳕MHC-1细胞质尾序列的分析表明，缺乏基于双亮氨酸和酪氨酸的内体信号转导基序，这些基序被认为有助于在鳕鱼中进行交叉表达。对鳕鱼信号转导基序，包括它们在细胞质尾部区域中的相对位置，进行更仔细的检查，表明这些基序可能是非功能性的，从而进一步支持了功能性研究以评估交叉展示的需要。最后，计算机分析和体外N型去糖基化实验表明，黑线鳕和鳕鱼β-2-微球蛋白（β2M）在相同的NQT序列中被糖基化。有趣的是，全基因组tBLASTn搜索还显示，假定的β2M糖基化位点频繁出现在Gadiformes谱系中，因为预测的NQT和其他NXS / T后代位于来自另外25个类腺基因组中的19个的β2M直向同源物中。尽管尚未阐明β2M糖基化的确切意义，但系统进化比较预测，相同的NQT糖基化序列还会在另外13个物种中发生，这些物种包括4个不同的放线菌纲（伞形目，大肠形目，芽孢形目和蠕形目）。这表明这个特征要么在多个谱系中独立出现，要么来自共同祖先，并在许多物种中丢失或修饰。一起，对适度MHC-1基因和β2M分子的分析突显了在最多样化的脊椎动物谱系（即鱼类）以及鱼类与研究程度更高的哺乳动物之间推广免疫系统范式的挑战。