Characterization of tumour-infiltrating lymphocytes in a tumour rejection cynomolgus macaque model.

Immunotherapy has emerged as a promising and effective treatment for cancer, yet the clinical benefit is still variable, in part due to insufficient accumulation of immune effector cells in the tumour microenvironment. Better understanding of tumour-infiltrating lymphocytes (TILs) from nonhuman primate tumours could provide insights into improving effector cell accumulation in tumour tissues during immunotherapy. Here, we characterize TILs in a cynomolgus macaque tumour model in which the tumours were infiltrated with CD4+ and CD8+ T cells and were eventually rejected. The majority of CD4+ and CD8+ TILs exhibited a CD45RA-CCR7- effector memory phenotype, but unlike circulating T cells, they expressed CD69, a marker for tissue-resident memory T (TRM) cells. CD69-expressing CD8+ TILs expressed high levels of the cytotoxic molecule granzyme B and the co-inhibitory receptor PD-1. Consistent with the TRM cell phenotype, CD8+ TILs minimally expressed CX3CR1 but expressed CXCR3 at higher levels than circulating CD8+ T cells. Meanwhile, CXCL9, CXCL10 and CXCL11, chemokine ligands for CXCR3, were expressed at high levels in the tumours, thus attracting CXCR3+CD8+ T cells. These results indicate that tumour-transplanted macaques can be a useful preclinical model for studying and optimizing T cell accumulation in tumours for the development of new immunotherapies.

Modulation of Igß is essential for the B cell selection in germinal center.

The positive and negative selection of antigen-reactive B cells take place in the germinal center (GC) during an immune responses. However, the precise molecular mechanisms underlying these selection machineries, including the involvement of antigen receptor signaling molecules, remain to be elucidated. We found that expression levels of Igα and Igß, which are the essential components of B cell antigen-receptor complex, were differentially regulated in GC B cells and that the expression of Igß was more prominently down-regulated in a portion of GC B cells. The suppression of Igß down-regulation reduced the number of GL7(+)GC B cells and the affinity maturation in T-dependent responses was markedly impaired. In addition, the disease phenotypes in autoimmune-prone mice were ameliorated by blocking of Igß down-regulation. These results suggest that Igß down-regulation is involved in the normal positive selection in GC and the accumulation of autoreactive B cells in autoimmune-prone mice.

IgG1 cytoplasmic tail is essential for cell surface expression in Igß down-regulated cells.

It has been shown that cytoplasmic tail of the IgG1 B cell receptors (BCRs) are essential for the induction of T-dependent immune responses. Also it has been revealed that unique tyrosine residue in the cytoplasmic tail of IgG2a has the potential of being phosphorylated at tyrosine and that this phosphorylation modulates BCR signaling. However, it still remains unclear whether such phosphorylation of IgG cytoplasmic tail is involved in the regulation of BCR surface expression. In order to approach the issue, we established and analyzed the cell lines which express wild-type or mutated forms of IgG1 BCR. As the result, we found that IgG1 BCR expressed normally on the surface of A20 B cell line independent of the cytoplasmic tail. In contrast, IgG1 BCR whose cytoplasmic tyrosine was replaced with glutamic acid which mimics phosphorylated tyrosine, was expressed most efficiently on the surface of non-B lineage cells and Igß-down-regulated B cell lines. These results suggest that tyrosine residue in IgG cytoplasmic tail is playing a essential role for the efficient expression of IgG BCR on the cell surface when BCR associated signaling molecules, including Igß, are down-regulated.

Activation-induced cytidine deaminase (AID)-dependent somatic hypermutation requires a splice isoform of the serine/arginine-rich (SR) protein SRSF1.

Somatic hypermutation (SHM) of Ig variable region (IgV) genes requires both IgV transcription and the enzyme activation-induced cytidine deaminase (AID). Identification of a cofactor responsible for the fact that IgV genes are much more sensitive to AID-induced mutagenesis than other genes is a key question in immunology. Here, we describe an essential role for a splice isoform of the prototypical serine/arginine-rich (SR) protein SRSF1, termed SRSF1-3, in AID-induced SHM in a DT40 chicken B-cell line. Unexpectedly, we found that SHM does not occur in a DT40 line lacking SRSF1-3 (DT40-ASF), although it is readily detectable in parental DT40 cells. Strikingly, overexpression of AID in DT40-ASF cells led to a large increase in nonspecific (off-target) mutations. In contrast, introduction of SRSF1-3, but not SRSF1, into these cells specifically restored SHM without increasing off-target mutations. Furthermore, we found that SRSF1-3 binds preferentially to the IgV gene and inhibits processing of the Ig transcript, providing a mechanism by which SRSF1-3 makes the IgV gene available for AID-dependent SHM. SRSF1 not only acts as an essential splicing factor but also regulates diverse aspects of mRNA metabolism and maintains genome stability. Our findings, thus, define an unexpected and important role for SRSF1, particularly for its splice variant, in enabling AID to function specifically on its natural substrate during SHM.

Efficient affinity maturation of antibodies in an engineered chicken B cell line DT40-SW by increasing point mutation.

The chicken B cell line DT40 undergoes hypermutation of immunoglobulin variable region (IgV) genes during culture, thereby constituting an antibody (Ab) library. We previously established an in vitro Ab generation system using an engineered line DT40-SW whose hypermutation machinery can be switched on and off. Abs for various antigens (Ags) can be obtained from the DT40-SW library and the specificity of the Ag-specific clones can be stabilized by stopping hypermutation. Furthermore, the affinity of obtained monoclonal Abs (mAbs) can be improved through further mutation followed by selection, a process analogous to "affinity maturation" that occurs in vivo. Although gene conversion dominantly diversifies the IgV genes in DT40 cells, point mutation is considered to be more favorable for fine-tuning Ab properties during affinity maturation. Here, we examined whether affinity maturation occurs more efficiently when the hypermutation pattern was transformed from gene conversion into point mutation in DT40-SW cells. To this end, we disrupted the XRCC3 gene that is essential for gene conversion. It was found that hemizygous disruption of the XRCC3 gene was sufficient to increase the point mutation frequency. Since hemizygous disruption is conducted more easily, we tested whether the XRCC3 (+/-) mutant generates high-affinity Abs through affinity maturation more efficiently than the wild type. Using this affinity maturation technique, we generated an improved 4-hydroxy-3-nitrophenylacetyl-specific mAb with approximately 600-fold lower K(D) than that of the original mAb. Taken together, hemizygous disruption of the XRCC3 gene is considered to be useful for obtaining high-affinity mAbs from DT40-SW cells though affinity maturation.

Enhancement of hypermutation frequency in the chicken B cell line DT40 for efficient diversification of the antibody repertoire.

Chicken B cell line DT40 continuously accumulates mutations in the immunoglobulin variable region (IgV) gene by gene conversion and point mutation, both of which are mediated by activation-induced cytidine deaminase (AID), thereby producing an antibody (Ab) library that is useful for screening monoclonal Abs (mAbs) in vitro. We previously generated an engineered DT40 line named DT40-SW, whose AID expression can be reversibly switched on or off, and developed an in vitro Ab generation system using DT40-SW cells. To efficiently create an Ab library with sufficient diversity, higher hypermutation frequency is advantageous. To this end, we generated a novel cell line DT40-SWDeltaC, which conditionally expresses a C-terminus-truncated AID mutant lacking the nuclear export signal. The transcription level of the mutant AID gene in DT40-SWDeltaC cells was similar to that of the wild-type gene in DT40-SW cells. However, the protein level of the truncated AID mutant was less than that of the wild type. The mutant protein was enriched in the nuclei of DT40-SWDeltaC cells, although the protein might be highly susceptible to degradation. In DT40-SWDeltaC cells, both gene conversion and point mutation occurred in the IgV gene with over threefold higher frequency than in DT40-SW cells, suggesting that a lower level of the mutant AID protein was sufficient to increase mutation frequency. Thus, DT40-SWDeltaC cells may be useful for constructing Ab libraries for efficient screening of mAbs in vitro.

Conditional transformation of immunoglobulin mutation pattern from gene conversion into point mutation by controlling XRCC3 expression in the DT40 B cell line.

A hypermutating B cell line DT40 is useful for screening antibodies and improving affinity of the selected antibodies in vitro. To perform affinity maturation efficiently, we generated an engineered DT40 line whose immunoglobulin mutation pattern can be transformed from gene conversion into point mutation by conditional suppression of XRCC3 expression.

Enhancement of antibody production from a chicken B cell line DT40 by reducing Pax5 expression.

We developed a novel in vitro antibody (Ab) generation system using a hypermutating chicken B cell line (DT40-SW). We suppressed the expression of the Pax5 transcription factor by targeted disruption of the gene to increase Ab production in isolated clones and produce the desired Abs. This single genetic manipulation resulted in a significant enhancement of Ab production without significantly affecting maximum cell density.

[Creation of valuable antibodies by an in vitro antibody generation system using a hypermutating B cell line].

Monoclonal antibodies (mAb) have recently proven to be excellent biopharmaceutical agents. The generation of hybridomas from antigen-stimulated B cells has been a key technology for obtaining mAbs; however, it is a laborious and time-consuming process, and sometimes mAbs for molecules conserved between species are difficult to obtain because of immunological tolerance. Thus, it is of great importance to develop in vitro technologies for generating useful Abs as drug candidates. We have been attempting to develop a novel in vitro antibody generation system using a chicken B cell line DT40, which displays Abs and mutates Ig genes during culture, thereby generating a useful Ab library for screening mAbs. First, we generated an engineered cell line DT40-SW whose mutation machinery can be reversibly switched on and off. The Ab generation system using DT40-SW is useful in the following ways: (1) mAbs for various model antigens including antoantigens can be obtained from the DT40-SW Ab library that is free from immunological tolerance; (2) the switching device of the mutation machinery enables fixing desirable Ig mutants by stopping mutation; (3) by repeated culture and sorting of clones bearing higher affinity for target antigens, affinity maturation can be mimicked in vitro. We have also genetically improved DT40-SW cells for mutation efficiency and Ab production. The Ab generation system will be applicable for obtaining valuable Abs such as antitumor Abs.

Loss of calcineurin homologous protein-1 in chicken B lymphoma DT40 cells destabilizes Na+/H+ exchanger isoform-1 protein.

NHE1/SLC9A1 is a ubiquitous isoform of vertebrate Na(+)/H(+) exchangers (NHEs) functioning in maintaining intracellular concentrations of Na(+) and H(+) ions. Calcineurin homologous protein-1 (CHP1) binds to the hydrophilic region of NHE1 and regulates NHE1 activity but reportedly does not play a role in translocating NHE1 from the endoplasmic reticulum to the plasma membrane. However, an antiport function of NHE1 requiring CHP1 remains to be clarified. Here we established CHP1-deficient chicken B lymphoma DT40 cells by gene targeting to address CHP1 function. CHP1-deficient cells showed extensive decreases in Na(+)/H(+) activities in intact cells. Although NHE1 mRNA levels were not affected, NHE1 protein levels were significantly reduced not only in the plasma membrane but in whole cells. The expression of a CHP1 transgene in CHP1-deficient cells rescued NHE1 protein expression. Expression of mutant forms of CHP1 defective in Ca(2+) binding or myristoylation also partially decreased NHE1 protein levels. Knockdown of CHP1 also caused a moderate decrease in NHE1 protein in HeLa cells. These data indicate that CHP1 primarily plays an essential role in stabilization of NHE1 for reaching of NHE1 to the plasma membrane and its exchange activity.

Novel in vitro screening system for monoclonal antibodies using hypermutating chicken B cell library.

Here, we report an in vitro screening system for monoclonal antibodies using a hypermutating chicken B cell line, DT40-SW. When switching on hypermutation, cultured DT40-SW cells constituted an antibody library, from which clones secreting antibodies to a test antigen were successfully isolated, and genetically stabilized by switching off the mutation machinery.

Genetic manipulation of an exogenous non-immunoglobulin protein by gene conversion machinery in a chicken B cell line.

During culture, a chicken B cell line DT40 spontaneously mutates immunoglobulin (Ig) genes by gene conversion, which involves activation-induced cytidine deaminase (AID)-dependent homologous recombination of the variable (V) region gene with upstream pseudo-V genes. To explore whether this mutation mechanism can target exogenous non-Ig genes, we generated DT40 lines that bears a gene conversion substrate comprising the green fluorescent protein (GFP) gene as a donor and the blue fluorescent protein (BFP) gene as an acceptor. A few percent of the initially BFP-expressing cells converted their fluorescence from blue to green after culture for 2-3 weeks when the substrate construct was integrated in the Ig light chain locus, but not in the ovalbumin locus. This was the result of AID-dependent and the GFP gene-templated gene conversion of the BFP gene, thereby leading to the introduction of various sizes of GFP-derived gene segment into the BFP gene. Thus, G/B construct may be used to visualize gene conversion events. After switching off AID expression in DT40 cells, the mutant clones were isolated stably and maintained with their mutations being fixed. Thus, the gene conversion machinery in DT40 cells will be a useful means to engineer non-Ig proteins by a type of DNA shuffling.

Reversible switching of immunoglobulin hypermutation machinery in a chicken B cell line.

A chicken B lymphoma line, DT40, hypermutates immunoglobulin (Ig) genes spontaneously during culture. Thus, cultured DT 40 cells constitute a useful Ig library for screening antibodies (Abs) in vitro. To fix desirable Ig mutants by stopping hypermutation or to resume mutation for further improvement of Ab affinity, activation-induced cytidine deaminase (AID), a key enzyme responsible for the Ig mutation machinery, must be switched on or off. To this end, we generated a DT40 line whose one AID allele was disrupted, and the other allele was replaced by the loxP-flanked AID construct. In this engineered cell line designated as DT40-SW, AID expression could be switched reversibly by tamoxifen-regulated Cre recombinase. Devices were also introduced to discriminate between the "AID-ON" and the "AID-OFF" cells by GFP expression and puromycin resistance, respectively. Starting from a single DT40-SW cell, Ig gene repertoire was efficiently diversified during culture only when AID expression was on.

B cell selection and affinity maturation during an antibody response in the mouse with limited B cell diversity.

The quasi-monoclonal mouse has limited B cell diversity, whose major (approximately 80%) B cell Ag receptors are comprised of the knockin V(H) 17.2.25 (V(H)T)-encoded H chain and the lambda1 or lambda2 L chain, thereby being specific for 4-hydroxy-3-nitrophenylacetyl. The p-nitrophenylacetyl (pNP) was found to be a low affinity analog of nitrophenylacetyl. We examined affinity maturation of anti-pNP IgG by analyzing mAbs obtained from quasi-monoclonal mice that were immunized with this low affinity Ag. The results are: 1) Although V(H)T/lambda1 and V(H)T/lambda2 IgM were equally produced, V(H)T/lambda2 IgG almost exclusively underwent affinity maturation toward pNP. 2) A common mutation in complementarity-determining region 3 of V(H)T (T313A) mainly contributed to generating the specificity for pNP. 3) Because mutated V(H)T-encoded gamma-chains could form lambda1-bearing IgG in Chinese hamster ovary cells, apparent absence of V(H)T/lambda1 anti-pNP IgG may not be due to the incompatibility between the gamma-chains and the lambda1-chain, but may be explained by the fact that V(H)T/lambda1 B cells showed 50- to 100-fold lower affinity for pNP than V(H)T/lambda2 B cells. 4) Interestingly, a pNP-specific IgM mAb that shared common mutations including T313A with high affinity anti-pNP IgG was isolated, suggesting that a part of hypermutation coupled with positive selection can occur before isotype switching. Thus, even weak B cell receptor engagement can elicit an IgM response, whereas only B cells that received signals stronger than a threshold may be committed to an affinity maturation process.