Two Forms of Adaptive Immunity in Vertebrates

Similarities and Differences

Masanori Kasahara 1; Yoichi Sutoh 2,    Department of Pathology, Hokkaido University Graduate School of Medicine, Sapporo, Japan
1 Corresponding author: email address: mkasaha@med.hokudai.ac.jp
2 Present address: Emory Vaccine Center and Department of Pathology and Laboratory Medicine, Emory University, Atlanta, Georgia 30322, USA

1 Introduction

Adaptive immunity is characterized by antigen-specific responses and memory. In jawed vertebrates, B-cell receptors (BCRs) and T-cell receptors (TCRs), expressed clonally on lymphocytes, play a central role for antigen recognition. These receptors generate diversity in their antigen-binding regions by somatically recombining variable (V) and joining (J), or V, diversity (D) and J gene fragments through DNA double-strand breaks mediated by the recombination-activating gene (RAG), thereby allowing the organism to have immune repertoires large enough to specifically recognize virtually any antigen (Schatz & Swanson, 2011; Tonegawa, 1983). Whereas BCRs, immunoglobulins (Igs: a secreted form of BCRs), and γδ TCRs can recognize antigens directly, αβ TCRs recognize antigens in the form of peptides bound to major histocompatibility complex (MHC) molecules (Blum, Wearsch, & Cresswell, 2013; Klein & Sato, 2000). Thus, the trios of BCR, TCR, and MHC molecules constitute the cornerstones of adaptive immunity.

Phylogenetically, all classes of jawed vertebrates have BCR, TCR, and MHC molecules (Flajnik & Kasahara, 2001, 2010; Kasahara, Suzuki, & DuPasquier, 2004; Litman, Rast, & Fugmann, 2010). Even the cartilaginous fish, the most primitive class of jawed vertebrates, have multiple Ig isotypes (Flajnik, 2002), TCRs of both αβ and γδ types (Rast et al., 1997) and polymorphic MHC class I and class II molecules (Hashimoto, Nakanishi, & Kurosawa, 1992; Kasahara, McKinney, Flajnik, & Ishibashi, 1993; Kasahara, Vazquez, Sato, McKinney, & Flajnik, 1992; Okamura, Ototake, Nakanishi, Kurosawa, & Hashimoto, 1997). Therefore, it appears that the BCR/TCR/MHC-based adaptive immune system (AIS) was established in a common ancestor of jawed vertebrates, and that once established, its basic architecture has been maintained virtually unchanged for the last 500 million years (Fig. 2.1).

Studies conducted in the 1960s and 1970s showed that jawless vertebrates represented by lampreys and hagfish were capable of producing specific agglutinins against particulate antigens and rejecting skin allografts with immunological memory (Acton, Weinheimer, Hildemann, & Evans, 1969; Finstad & Good, 1964; Fujii, Nakagawa, & Murakawa, 1979; Linthicum & Hildemann, 1970; Litman, Finstad, Howell, Pollara, & Good, 1970; Marchalonis & Edelman, 1968; Pollara, Litman, Finstad, Howell, & Good, 1970), suggesting that the origin of adaptive immunity can be traced back to the emergence of jawless vertebrates. However, extensive efforts by immunologists to identify BCR, TCR, or MHC molecules in lampreys or hagfish ended in vain. This was quite puzzling and even cast doubt on the credibility of the earlier observations pointing to the existence of adaptive immunity in jawless vertebrates. Ultimately, this puzzle was resolved by the groundbreaking discovery that, instead of BCRs and TCRs, lampreys use a unique antigen receptor now known as variable lymphocyte receptors (VLR) (Pancer, Amemiya, et al., 2004).

VLRs are members of the leucine-rich repeat (LRR) family of proteins and thus are structurally unrelated to BCRs or TCRs. Nevertheless, they can generate diversity comparable to that of gnathostome antigen receptors by assembling variable LRR modules. The discovery of VLRs in lampreys (Pancer, Amemiya, et al., 2004) and subsequently in hagfish (Pancer et al., 2005) demonstrated that jawless vertebrates have a unique form of adaptive immunity that does not rely on BCR, TCR, or MHC molecules (Cooper & Alder, 2006; Hirano, Das, Guo, & Cooper, 2011; Pancer & Cooper, 2006), thus highlighting the difference in the AISs of jawed and jawless vertebrates. However, accumulating evidence indicates that the two forms of adaptive immunity also have much in common (Boehm, 2011; Boehm, Iwanami, & Hess, 2012). Most notable is the apparent conservation of lymphocyte lineages (Guo et al., 2009).

We review here the current knowledge on the AIS of jawless vertebrates, emphasizing both the similarities to and differences from the AIS of jawed vertebrates.

2 Discovery of VLR

In the early 2000s, transcriptome analysis of lamprey lymphocytes was conducted to search for genes involved in adaptive immunity (Mayer, Uinuk-Ool, et al., 2002; Uinuk-Ool et al., 2002). As described earlier, this analysis failed to identify BCRs, TCRs, or MHC molecules. Instead, in the course of searching for genes whose expression is upregulated in antigen-stimulated lymphocytes, Pancer and his colleagues identified a large number of transcripts encoding a variable number of LRR modules (Pancer, Amemiya, et al., 2004). Notably, the number and the sequences of LRR modules were highly variable, but the sequences flanking the modules were completely invariant, raising the possibility that all of the transcripts were derived from a single gene. This possibility was confirmed by genomic analysis, which showed that the lamprey genome contains only a single copy of this gene. Surprisingly, the organization of this gene differed between lymphocytes and other somatic cells. In the latter, the gene had an incomplete structure lacking LRR-encoding modules, and many LRR-encoding modules were located in its vicinity. In lymphocytes, however, the gene had a structure with LRR-encoding modules in between the invariant 5′- and 3′-sequences (Pancer, Amemiya, et al., 2004), indicating that gene assembly took place exclusively in lymphocytes. These observations suggested strongly that this gene, designated VLR (now known as VLRB), codes for a long-sought antigen receptor of jawless vertebrates. This suggestion was soon confirmed experimentally by Cooper and his colleagues (Alder et al., 2005).

Hagfish is another surviving member of jawless vertebrates assumed to have diverged from lampreys 450–500 million years ago (Kuraku & Kuratani, 2006; Shimeld & Donoghue, 2012). Shortly after the discovery of the lamprey VLR, an expressed sequence tag database of hagfish leukocytes (Suzuki, Shin-I, Kohara, & Kasahara, 2004) was searched to examine whether hagfish have VLR. This search resulted in the identification of two VLR genes (Pancer et al., 2005). One gene corresponded to the VLR gene discovered in the lamprey, and another was a novel VLR gene. These genes were named VLRB and VLRA, respectively. Subsequently, a gene thought to be orthologous to VLRA was also identified in lampreys, thus establishing that both lampreys and hagfish have two VLR genes (Rogozin et al., 2007).

Recently, a third VLR gene, named VLRC, was identified in the lamprey (Kasamatsu et al., 2010). More recently, a third...