Introduction to HLA and MHC
An HLA (Human Leukocyte Antigen) or human MHC (Major Histocompatibility Complex) is a group of surface protein complexes that presents antigens to downstream immune cells to activate the adaptive immune system against foreign pathogens or injured cells. These molecules are critical for modulating T cell responses and play a major role in several autoimmune diseases including diabetes, arthritis, and multiple sclerosis. The HLA protein family is one of the most polymorphic gene families within the human genome. For classification purposes, researchers divide these complexes into two main groups, each with different characteristics: HLA class I and HLA class II (Table 1).| HLA Class I | HLA Class II | |
Alleles | A, B, C | DP, DQ, DR |
Location | All nucleated cells | Antigen-presenting cells (APCs) |
Structure | Single chain consisting of α1, α2, α3 domains, coupled with B2m | Heterologous chain consisting of α1, β1, and α2, β2 |
Antigen | Endogenous | Exogenous |
T cells Activated | CD8+ cells | CD4+ cells |
Table 1: Summary of differences between HLA Class I and Class II Complexes.
Taconic's Chimeric HLA Mice Allow Identification of HLA-Restricted Epitopes
MHC molecules found in mice do not necessarily present the same antigens as HLA molecules. Therefore, in order to enable the study of HLA epitopes, HLA transgenic mice were generated1-4. These models have allowed researchers to study HLA-restricted T cell responses in vivo for infectious disease, vaccine, and autoimmune research. Mice expressing full-length HLA showed some initial promise in the identification of HLA-restricted responses following immunization with viral antigens5. Nevertheless, these mice suffered from inefficient HLA-restricted T cell responses, which is believed to be a result of HLA not being recognized effectively by murine T cell receptors (TCRs)6.To increase the efficiency of HLA recognition by murine T cells, Taconic Biosciences provides chimeric HLA mice with a humanized antigen-binding domain coupled with murine MHC domains that activate downstream mouse T cells (Table 2). This ensures that the specificity of HLA-restricted epitope presentation is not compromised in chimeric HLA-expressing cells while still allowing for efficient downstream activation of CD4 or CD8 T cells (Figure 1).
Figure 1: Taconic's HLA mice express human (grey) HLA domains to ensure HLA-restricted antigen presentation while murine MHC (cyan) domains result in efficient binding to murine CD8/4 and subsequent downstream T cell activation.
Read the Related Taconic Insight:Understanding the Relationship Between Murine MHC Expression and HLA Restriction in Taconic's HLA models
Taconic's class I HLA mice were originally obtained from scientists at Pharmexa-Epimmune. These strains are offered on two genetic backgrounds, CB6F1 and C57BL/6. Mice on the CB6F1 strain background are generated by crossing homozygous HLA C57BL/6 with wild type BALB/cAnNTac mice. Mice on an F1 background may have a better HLA-restricted immune response than mice on a pure B6 background. It has been shown that loss of murine MHC leads to reduction in T cell diversity and population numbers7,8. Thus, presence of BALB/c and C57BL/6 murine H2 in CB6F1 offspring may help maintain a sufficiently large and diverse naïve T cell population in comparison to HLA mice of a pure B6 background and in comparison to other transgenic class I HLA models on a murine MHC knockout background which may have reduced T cell numbers and diversity. CB6F1 mice display hybrid vigor and display fewer health issues compared to inbred parental strains since they are heterozygous for all autosomal loci that differ between C57BL/6 and BALB/c genomes and thus lack many of the inherent idiosyncrasies of either parental line. HLA mice on a pure B6 background may be preferred for syngeneic tumor studies and other applications. Consultation with experienced PhD-level scientists is recommended to determine which version is most appropriate for your studies.In contrast to HLA class I mice, Taconic's HLA class II HLA-DR4 mice do not express murine MHC, which is believed to improve the utilization of HLA-DR4 as a restricting element. The absence of murine class II MHC seems necessary to elicit sufficient HLA-class II restricted response9, while the lack of murine MHC is not necessary for proper function of class I HLA mice. Hence, HLA-DR4 animals are provided on a pure B6 background. Despite these differences in strain backgrounds between the class I and class II transgenic mice, both groups still show HLA-restricted response to a variety of infections10-13.
| HLA CLASS I | ||||
|---|---|---|---|---|
| MODEL | STRAIN BACKGROUND | DESCRIPTION | MGI | |
| HLA-A2.1 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-A2.1 class I molecule (representing the HLA-A2 supertype) on the surface of T and B cells. | Link | |
| HLA-A11 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-A11 class I molecule (representing the HLA-A3 supertype) on the surface of T and B cells. | Link | |
| HLA-A1 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-A1 class I molecule (representing the HLA-A1 supertype) on the surface of T and B cells. | Link | |
| HLA-A24 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-A24 class I molecule (representing the HLA-A24 supertype) on the surface of T and B cells. | Link | |
| HLA-B7 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-B7 class I molecule (representing the HLA-B7 supertype) on the surface of T and B cells. | Link | |
| HLA-B44 | C57BL/6 or CB6F1 | Expresses the chimeric HLA-B44 class I molecule (representing the HLA-B44 supertype) on the surface of T and B cells. | Link | |
| HLA CLASS II | ||||
| Abb Knockout/ Transgenic HLA-DR4 | C57BL/6 | Expresses the chimeric HLA-DR4 class II molecule on the surface of T and B cells. | Link | |
Table 2: Summary of different HLA mice available exclusively from Taconic.
Not All HLA Mice Perform the Same
It is important to note that not all HLA-related mouse models perform the same when it comes to HLA restriction. HLA mice can differ based on:- Chimeric or fully-humanized HLA
- HLA gene copy number
- HLA transgene insertion sites
- Mouse MHC expressed or knocked out
- Background strain differences
Taconic's HLA mice have been used to study human class I- or class II-restricted T cells in autoimmune diseases, infectious diseases, and vaccine development. As valuable tools in evaluating human HLA restricted T cell-mediated vaccine efficacy in oncology, they also offer researchers greater translatability in studying immune responses.
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