Visualization of immune pathways that enhance the neutralizing antibody response to vaccines after primary immunization

We examined the relationship between the association of a vaccine antigen with immune cells in secondary lymphoid organs shortly after immunization and the resulting neutralizing antibody response induced by that antigen using three antigenic forms of anthrax protective antigen (PA) that induce qualitatively different antibody responses. The three PA forms used were wild-type PA, which binds to anthrax toxin receptors and elicits a robust antibody response that includes both neutralizing and non-neutralizing antibodies; a receptor-binding-deficient (RBD) mutant form of PA, which does not bind cellular receptors and elicits only barely detectable antibody responses; and an engineered chimeric form of PA, which binds cholera toxin receptors and elicits a robust total antibody response but a poor neutralizing antibody response. We found that both wild-type PA and the PA chimera associated with immune cells in secondary lymphoid organs after immunization, but the RBD mutant PA exhibited minimal association, revealing a relationship between antigen binding to toxin receptors on immune cells after immunization and subsequent antibody responses. A portion of wild-type PA that bound to immune cells was cell surface-associated and maintained its native conformation. Much lower amounts of conformationally intact PA chimera were associated with immune cells after immunization, correlating with the lower neutralizing antibody response elicited by the PA chimera. Thus, binding of an antigen to receptors on immune cells in secondary lymphoid organs after immunization and maintenance of conformational integrity of the cell-associated antigen help dictate the magnitude of the resulting neutralizing antibody response, but not necessarily the total antibody response.IMPORTANCEMany vaccines protect by the induction of antibodies that neutralize the action of the pathogen. Here, we followed the fate of three antigenic forms of a vaccine antigen in secondary lymphoid organs after immunization to investigate events leading to a robust neutralizing antibody response. We found that the magnitude of the neutralizing antibody response, but not the total antibody response, correlates with the levels of conformationally intact antigen associated with immune cells in secondary lymphoid organs after primary immunization. We believe that these results provide important insights into the genesis of neutralizing antibody responses induced by vaccine antigens and may have implications for vaccine design.

In vivo and in vitro immune responses against Francisella tularensis vaccines are comparable among Fischer 344 rat substrains

Identifying suitable animal models and standardizing preclinical methods are important for the generation, characterization, and development of new vaccines, including those against Francisella tularensis. Non-human primates represent an important animal model to evaluate tularemia vaccine efficacy, and the use of correlates of vaccine-induced protection may facilitate bridging immune responses from non-human primates to people. However, among small animals, Fischer 344 rats represent a valuable resource for initial studies to evaluate immune responses, to identify correlates of protection, and to screen novel vaccines. In this study, we performed a comparative analysis of three Fischer rat substrains to determine potential differences in immune responses, to evaluate methods used to quantify potential correlates of protection, and to evaluate protection after vaccination. To this end, we took advantage of data previously generated using one of the rat substrains by evaluating two live vaccines, LVS and F. tularensis SchuS4-ΔclpB (ΔclpB). We compared immune responses after primary vaccination, adaptive immune responses upon re-stimulation of leukocytes in vitro, and sensitivity to aerosol challenge. Despite some detectable differences, the results highlight the similarity of immune responses to tularemia vaccines and challenge outcomes between the three substrains, indicating that all offer acceptable and comparable approaches as animal models to study Francisella infection and immunity.

IL-12p40 is essential but not sufficient for Francisella tularensis LVS clearance in chronically infected mice

IL-12p40 plays an important role in F. tularensis Live Vaccine Strain (LVS) clearance that is independent of its functions as a part of the heterodimeric cytokines IL-12p70 or IL-23. In contrast to WT, p35, or p19 knockout (KO) mice, p40 KO mice infected with LVS develop a chronic infection that does not resolve. Here, we further evaluated the role of IL-12p40 in F. tularensis clearance. Despite reduced IFN-γ production, primed splenocytes from p40 KO and p35 KO mice appeared functionally similar to those from WT mice during in vitro co-culture assays of intramacrophage bacterial growth control. Gene expression analysis revealed a subset of genes that were upregulated in re-stimulated WT and p35 KO splenocytes, but not p40 KO splenocytes, and thus are candidates for involvement in F. tularensis clearance. To directly evaluate a potential mechanism for p40 in F. tularensis clearance, we reconstituted protein levels in LVS-infected p40 KO mice using either intermittent injections of p40 homodimer (p80) or treatment with a p40-producing lentivirus construct. Although both delivery strategies yielded readily detectable levels of p40 in sera and spleens, neither treatment had a measurable impact on LVS clearance by p40 KO mice. Taken together, these studies demonstrate that clearance of F. tularensis infection depends on p40, but p40 monomers and/or dimers alone are not sufficient.

Deficiency in CCR2 increases susceptibility of mice to infection with an intracellular pathogen, Francisella tularensis LVS, but does not impair development of protective immunity

CCR2 is the major chemokine receptor that regulates appropriate trafficking of inflammatory monocytes, but the role of this chemokine receptor and its ligands during primary and secondary infection with intracellular infections remains incompletely understood. Here we used murine infection with the Live Vaccine Strain (LVS) of Francisella tularensis to evaluate the role of CCR2 during primary and secondary parenteral responses to this prototype intracellular bacterium. We find that mice deficient in CCR2 are highly compromised in their ability to survive intradermal infection with LVS, indicating the importance of this receptor during primary parenteral responses. Interestingly, this defect could not be readily attributed to the activities of the known murine CCR2 ligands MCP-1/CCL2, MCP-3/CCL7, or MCP-5/CCL12. Nonetheless, CCR2 knockout mice vaccinated by infection with low doses of LVS generated optimal T cell responses that controlled the intramacrophage replication of Francisella, and LVS-immune CCR2 knockout mice survived maximal lethal Francisella challenge. Thus, fully protective adaptive immune memory responses to this intracellular bacterium can be readily generated in the absence of CCR2.

Production of IFN-γ by splenic dendritic cells during innate immune responses against Francisella tularensis LVS depends on MyD88, but not TLR2, TLR4, or TLR9

Production of IFN-γ is a key innate immune mechanism that limits replication of intracellular bacteria such as Francisella tularensis (Ft) until adaptive immune responses develop. Previously, we demonstrated that the host cell types responsible for IFN-γ production in response to murine Francisella infection include not only natural killer (NK) and T cells, but also a variety of myeloid cells. However, production of IFN-γ by mouse dendritic cells (DC) is controversial. Here, we directly demonstrated substantial production of IFN-γ by DC, as well as hybrid NK-DC, from LVS-infected wild type C57BL/6 or Rag1 knockout mice. We demonstrated that the numbers of conventional DC producing IFN-γ increased progressively over the course of 8 days of LVS infection. In contrast, the numbers of conventional NK cells producing IFN-γ, which represented about 40% of non-B/T IFN-γ-producing cells, peaked at day 4 after LVS infection and declined thereafter. This pattern was similar to that of hybrid NK-DC. To further confirm IFN-γ production by infected cells, DC and neutrophils were sorted from naïve and LVS-infected mice and analyzed for gene expression. Quantification of LVS by PCR revealed the presence of Ft DNA not only in macrophages, but also in highly purified, IFN-γ producing DC and neutrophils. Finally, production of IFN-γ by infected DC was confirmed by immunohistochemistry and confocal microscopy. Notably, IFN-γ production patterns similar to those in wild type mice were observed in cells derived from LVS-infected TLR2, TLR4, and TLR2xTLR9 knockout (KO) mice, but not from MyD88 KO mice. Taken together, these studies demonstrate the pivotal roles of DC and MyD88 in IFN-γ production and in initiating innate immune responses to this intracellular bacterium.

A panel of correlates predicts vaccine-induced protection of rats against respiratory challenge with virulent Francisella tularensis

There are no defined correlates of protection for any intracellular pathogen, including the bacterium Francisella tularensis, which causes tularemia. Evaluating vaccine efficacy against sporadic diseases like tularemia using field trials is problematic, and therefore alternative strategies to test vaccine candidates like the Francisella Live Vaccine Strain (LVS), such as testing in animals and applying correlate measurements, are needed. Recently, we described a promising correlate strategy that predicted the degree of vaccine-induced protection in mice given parenteral challenges, primarily when using an attenuated Francisella strain. Here, we demonstrate that using peripheral blood lymphocytes (PBLs) in this approach predicts LVS-mediated protection against respiratory challenge of Fischer 344 rats with fully virulent F. tularensis, with exceptional sensitivity and specificity. Rats were vaccinated with a panel of LVS-derived vaccines and subsequently given lethal respiratory challenges with Type A F. tularensis. In parallel, PBLs from vaccinated rats were evaluated for their functional ability to control intramacrophage Francisella growth in in vitro co-culture assays. PBLs recovered from co-cultures were also evaluated for relative gene expression using a large panel of genes identified in murine studies. In vitro control of LVS intramacrophage replication reflected the hierarchy of protection. Further, despite variability between individuals, 22 genes were significantly more up-regulated in PBLs from rats vaccinated with LVS compared to those from rats vaccinated with the variant LVS-R or heat-killed LVS, which were poorly protective. These genes included IFN-γ, IL-21, NOS2, LTA, T-bet, IL-12rβ2, and CCL5. Most importantly, combining quantifications of intramacrophage growth control with 5-7 gene expression levels using multivariate analyses discriminated protected from non-protected individuals with greater than 95% sensitivity and specificity. The results therefore support translation of this approach to non-human primates and people to evaluate new vaccines against Francisella and other intracellular pathogens.

Utilizing previously identified in vitro correlates of protection to predict the efficacy of a novel vaccine candidate against the intracellular bacterium Francisella tularensis

Francisella tularensis (Ft) is an intracellular bacterium that causes tularemia, a disease with a low incidence in US. The only available vaccine, the Live Vaccine Strain (LVS), is investigational and is derived from Type B Ft, not the more virulent Type A Ft. Previous work produced potential correlates to predict successful vaccination. These were determined by in vitro stimulation of murine Ft LVS-immune cells and analyses of their gene expression. We used this approach to investigate correlates of protection for the novel ΔclpB vaccine, derived from Type A Ft SchuS4. Mice were vaccinated with ΔclpB as well as LVS-derived vaccines and subsequently challenged with a lethal dose of LVS, after which all mice vaccinated with ΔclpB and LVS survived. The in vivo survival data was compared with in vitro data obtained from PBLs and splenocytes from vaccinated mice. In general, the in vitro functions of leukocytes from ΔclpB-vaccinated mice were comparable or exceeded those of leukocytes from LVS-vaccinated mice, including control of LVS intramacrophage replication, IFN-gamma secretion, and NO production. Correlates up-regulated in cells from mice vaccinated with ΔclpB included IFN-gamma, IL-21, Nos2, LTA, T-bet, IL-12rbeta2, CCL5 and GzmB; in some cases up-regulation was higher than that from LVS-derived PBLs. Other genes were up-regulated in ΔclpB-derived but not LVS-derived leukocytes, suggesting that improved protection stimulated by the ΔclpB vaccine may be related to the change in strain and/or to stronger immune responses. Hence, this panel of correlates could contribute to the screening of new vaccine candidates, bridging from animal models to humans, and augmenting clinical trials.

GM-CSF has disparate roles during intranasal and intradermal Francisella tularensis infection

Our laboratory has employed in vitro and in vivo mouse models based on Francisella tularensis Live Vaccine Strain (LVS)-induced protection to elucidate immune correlates for intracellular bacteria. Among the effectors found was GM-CSF, a pleiotropic cytokine that is integral to the development and proliferation of myeloid cells, including alveolar macrophages. GM-CSF has roles in resistance to primary murine infection with several intracellular pathogens, but its role during Francisella infection is unknown. Francisella is an intracellular pathogen that infects lungs after inhalation, primarily invading alveolar macrophages. Here we show that GM-CSF has route-dependent roles during primary infection of mice with LVS. GM-CSF deficient (GM-CSF KO) mice were slightly more susceptible than wild type to intradermal infection, but had increased resistance to intranasal infection. Similarly, these mice had increased resistance to pulmonary infection with virulent F. tularensis (SchuS4). LVS-vaccinated GM-CSF KO mice had normal adaptive immune responses, as measured by T cell activities after LVS intradermal or intranasal vaccination, and survived lethal secondary LVS challenge. GM-CSF KO mice also had robust humoral responses, producing elevated levels of serum antibodies following LVS vaccination compared to wild type mice. Taken together, our data demonstrates that the absence of GM-CSF improves resistance to pulmonary, but not intradermal, infection with Francisella.

Progress, challenges, and opportunities in Francisella vaccine development

Renewed interest in Francisella tularensis has resulted in substantial new information about its pathogenesis and immunology, along with development of useful animal models. While understanding of protective immunity against Francisella remains incomplete, data in both animals and humans suggest that inducing T cell-mediated immunity is crucial for successful vaccination with current candidates such as the Live Vaccine Strain (LVS), with specific antibodies and immune B cells playing supporting roles. Consistent with this idea, recent results indicate that measurements of T cell functions and relative gene expression by immune T cells predict vaccine-induced protection in animal models. Because field trials of new vaccines will be difficult to design, using such measurements to derive potential correlates of protection may be important to bridge between animal efficacy studies and people.

Francisella tularensis Vaccines Elicit Concurrent Protective T- and B-Cell Immune Responses in BALB/cByJ Mice

In the last decade several new vaccines against Francisella tularensis, which causes tularemia, have been characterized in animal models. Whereas many of these vaccine candidates showed promise, it remains critical to bridge the preclinical studies to human subjects, ideally by taking advantage of correlates of protection. By combining in vitro intramacrophage LVS replication with gene expression data through multivariate analysis, we previously identified and quantified correlative T cell immune responses that discriminate vaccines of different efficacy. Further, using C57BL/6J mice, we demonstrated that the relative levels of gene expression vary according to vaccination route and between cell types from different organs. Here, we extended our studies to the analysis of T cell functions of BALB/cByJ mice to evaluate whether our approach to identify correlates of protection also applies to a Th2 dominant mouse strain. BALB/cByJ mice had higher survival rates than C57BL/6J mice when they were immunized with suboptimal vaccines and challenged. However, splenocytes derived from differentially vaccinated BALB/cByJ mice controlled LVS intramacrophage replication in vitro in a pattern that reflected the hierarchy of protection observed in C57BL/6J mice. In addition, gene expression of selected potential correlates revealed similar patterns in splenocytes of BALB/cByJ and C57BL/6J mice. The different survival patterns were related to B cell functions, not necessarily to specific antibody production, which played an important protective role in BALB/cByJ mice when vaccinated with suboptimal vaccines. Our studies therefore demonstrate the range of mechanisms that operate in the most common mouse strains used for characterization of vaccines against F. tularensis, and illustrate the complexity necessary to define a comprehensive set of correlates.

Models derived from in vitro analyses of spleen, liver, and lung leukocyte functions predict vaccine efficacy against the Francisella tularensis Live Vaccine Strain (LVS)

Currently, there are no licensed vaccines and no correlates of protection against Francisella tularensis, which causes tularemia. We recently demonstrated that measuring in vitro control of intramacrophage bacterial growth by murine F. tularensis-immune splenocytes, as well as transcriptional analyses, discriminated Francisella vaccines of different efficacies. Further, we identified potential correlates of protection against systemic challenge. Here, we extended this approach by studying leukocytes derived from lungs and livers of mice immunized by parenteral and respiratory routes with F. tularensis vaccines. Liver and lung leukocytes derived from intradermally and intranasally vaccinated mice controlled in vitro Francisella Live Vaccine Strain (LVS) intramacrophage replication in patterns similar to those of splenocytes. Gene expression analyses of potential correlates also revealed similar patterns in liver cells and splenocytes. In some cases (e.g., tumor necrosis factor alpha [TNF-α], interleukin 22 [IL-22], and granulocyte-macrophage colony-stimulating factor [GM-CSF]), liver cells exhibited even higher relative gene expression, whereas fewer genes exhibited differential expression in lung cells. In contrast with their strong ability to control LVS replication, splenocytes from intranasally vaccinated mice expressed few genes with a hierarchy of expression similar to that of splenocytes from intradermally vaccinated mice. Thus, the relative levels of gene expression vary between cell types from different organs and by vaccination route. Most importantly, because studies comparing cell sources and routes of vaccination supported the predictive validity of this coculture and gene quantification approach, we combined in vitro LVS replication with gene expression data to develop analytical models that discriminated between vaccine groups and successfully predicted the degree of vaccine efficacy. Thus, this strategy remains a promising means of identifying and quantifying correlative T cell responses. IMPORTANCE Identifying and quantifying correlates of protection is especially challenging for intracellular bacteria, including Francisella tularensis. F. tularensis is classified as a category A bioterrorism agent, and no vaccines have been licensed in the United States, but tularemia is a rare disease. Therefore, clinical trials to test promising vaccines are impractical. In this report, we further evaluated a novel approach to developing correlates by assessing T cell immune responses in lungs and livers of differentially vaccinated mice; these nonprofessional immune tissues are colonized by Francisella. The relative degree of vaccine efficacy against systemic challenge was reflected by the ability of immune T cells, particularly liver T cells, to control the intramacrophage replication of bacteria in vitro and by relative gene expression of several immunological mediators. We therefore developed analytical models that combined bacterial replication data and gene expression data. Several resulting models provided excellent discrimination between vaccines of different efficacies.

Development of functional and molecular correlates of vaccine-induced protection for a model intracellular pathogen, F. tularensis LVS

In contrast with common human infections for which vaccine efficacy can be evaluated directly in field studies, alternative strategies are needed to evaluate efficacy for slowly developing or sporadic diseases like tularemia. For diseases such as these caused by intracellular bacteria, serological measures of antibodies are generally not predictive. Here, we used vaccines varying in efficacy to explore development of clinically useful correlates of protection for intracellular bacteria, using Francisella tularensis as an experimental model. F. tularensis is an intracellular bacterium classified as Category A bioterrorism agent which causes tularemia. The primary vaccine candidate in the U.S., called Live Vaccine Strain (LVS), has been the subject of ongoing clinical studies; however, safety and efficacy are not well established, and LVS is not licensed by the U.S. FDA. Using a mouse model, we compared the in vivo efficacy of a panel of qualitatively different Francisella vaccine candidates, the in vitro functional activity of immune lymphocytes derived from vaccinated mice, and relative gene expression in immune lymphocytes. Integrated analyses showed that the hierarchy of protection in vivo engendered by qualitatively different vaccines was reflected by the degree of lymphocytes' in vitro activity in controlling the intramacrophage growth of Francisella. Thus, this assay may be a functional correlate. Further, the strength of protection was significantly related to the degree of up-regulation of expression of a panel of genes in cells recovered from the assay. These included IFN-γ, IL-6, IL-12Rβ2, T-bet, SOCS-1, and IL-18bp. Taken together, the results indicate that an in vitro assay that detects control of bacterial growth, and/or a selected panel of mediators, may ultimately be developed to predict the outcome of vaccine efficacy and to complement clinical trials. The overall approach may be applicable to intracellular pathogens in general.

Diseases such as tuberculosis (caused by Mycobacterium tuberculosis) or tularemia (caused by Francisella tularensis) result from infections by microbes that live within cells of a person's body. New vaccines are being developed against such intracellular pathogens, but some will be difficult to test, because disease takes a long time to develop (e.g., tuberculosis) or because outbreaks are unpredictable (e.g., tularemia). Usually such infections are controlled by activities of T cells. However, there are no accepted measures of T cell function that reliably predict vaccine-induced protection. We studied two new ways to do so. We used a group of vaccine candidates against tularemia that stimulated good, fair, or poor protection of mice against Francisella challenge. We then measured whether Francisella–immune cells from vaccinated mice controlled the growth of bacteria inside cells, and/or whether the expression of immune genes in Francisella–immune cells was increased. We found that the degree of protection was matched by the degree of the cells' function in controlling intramacrophage bacterial growth. Further, the degree was predicted by relative amounts of gene expression for several immune mediators. Thus the two new options explored here may help predict protection, without waiting for the onset of disease.

NK cells activated in vivo by bacterial DNA control the intracellular growth of Francisella tularensis LVS

We demonstrated previously that mice treated with bacterial or oligonucleotide DNA containing unmethylated CpG motifs are transiently protected against lethal parenteral challenge with the intracellular bacterium Francisella tularensis Live Vaccine Strain (LVS). Here we explore the cellular basis of this protection. Wild-type mice that were treated with CpG oligonucleotide DNA and challenged with a lethal dose of LVS survived, while mice lacking TLR9 did not. In vitro, treatment of LVS-infected macrophages and/or naive splenocytes with oligo DNA had no impact on intracellular bacterial replication. In contrast, in vitro co-culture of LVS-infected macrophages with splenocytes obtained from mice treated with oligo DNA in vivo resulted in control of intracellular LVS growth. Control was reversed by antibodies to interferon-gamma or to tumor necrosis factor-alpha and by inhibition of nitric oxide, and to a lesser degree by antibodies to Interleukin-12. Further, splenocytes from DNA-primed normal, T cell KO, B cell KO, lymphocyte-deficient scid, or perforin KO mice all controlled intra-macrophage LVS growth. Enriched DNA-primed natural killer cells, but not B cells, clearly controlled intracellular LVS growth. Thus, NK cells contribute to DNA-mediated protection by production of cytokines including IFN-gamma and TNF-alpha, resulting in nitric oxide production and control of intracellular Francisella replication.

SDR grafting--a new approach to antibody humanization

A major impediment to the clinical utility of the murine monoclonal antibodies is their potential to elicit human anti-murine antibody (HAMA) response in patients. To circumvent this problem, murine antibodies have been genetically manipulated to progressively replace their murine content with the amino acid residues present in their human counterparts. To that end, murine antibodies have been humanized by grafting their complementarity determining regions (CDRs) onto the variable light (V(L)) and variable heavy (V(H)) frameworks of human immunoglobulin molecules, while retaining those murine framework residues deemed essential for the integrity of the antigen-combining site. However, the xenogeneic CDRs of the humanized antibodies may evoke anti-idiotypic (anti-Id) response in patients. To minimize the anti-Id response, a procedure to humanize xenogeneic antibodies has been described that is based on grafting, onto the human frameworks, only the specificity determining residues (SDRs), the CDR residues that are most crucial in the antibody-ligand interaction. The SDRs are identified through the help of the database of the three-dimensional structures of the antigen-antibody complexes of known structures or by mutational analysis of the antibody-combining site. An alternative approach to humanization, which involves retention of more CDR residues, is based on grafting of the 'abbreviated' CDRs, the stretches of CDR residues that include all the SDRs. A procedure to assess the reactivity of the humanized antibody to sera from patients who had been administered the murine antibody has also been described.

Minimizing the Immunogenicity of Antibodies for Clinical Application

The clinical utility of murine monoclonal antibodies has been greatly limited by the human anti-murine antibody responses they effect in patients. To make them less immunogenic, murine antibodies have been genetically engineered to progressively replace their murine content with that of their human counterparts. This review describes the genetic approaches that have been used to humanize murine antibodies, including the generation of mouse-human chimeric antibodies, veneering of the mouse variable regions, and the grafting of murine complementarity-determining regions (CDRs) onto the variable light (VL) and variable heavy (VH) frameworks of human immunoglobulin molecules, while retaining only those murine framework residues deemed essential for the integrity of the antigen-binding site. To minimize the anti-idiotypic responses that could still be evoked by the murine CDRs in humanized antibodies, two approaches have also been described. These are based on grafting onto the human frameworks the 'abbreviated' CDRs or only the specificity-determining residues (SDRs), the CDR residues that are involved in antigen interaction. The SDRs are identified through the help of the database of three-dimensional structures of antibody:antigen complexes or by mutational analysis of the antibody-combining site. In addition, we also describe the use of in vitro affinity maturation to enhance the binding affinity of humanized antibodies, as well as the manipulation of framework residues to maximize their human content and minimize their immunogenic potential.

Inhibition of CD4(+)25+ T regulatory cell function implicated in enhanced immune response by low-dose cyclophosphamide

Regulatory T cells (T(REGs)) control the key aspects of tolerance and play a role in the lack of antitumor immune responses. Cyclophosphamide (CY) is a chemotherapeutic agent with a dose-dependent, bimodal effect on the immune system. Although a previous study demonstrated that CY reduces the number of T(REGs), the mechanism involved in this process has yet to be defined. In this report, it is established that low-dose CY not only decreases cell number but leads to decreased functionality of T(REGs). CY treatment enhances apoptosis and decreases homeostatic proliferation of these cells. Expression of GITR and FoxP3, which are involved in the suppressive activity of T(REGs), is down-regulated after CY administration, though the level of expression varies depending on the time studied. This is the first report demonstrating that CY, in addition to decreasing cell number, inhibits the suppressive capability of T(REGs). The relevance of the loss of suppressor functionality and the changes in gene expression are further discussed.

SDR grafting of a murine antibody using multiple human germline templates to minimize its immunogenicity

The humanization of mAbs by complementarity-determining region (CDR)-grafting has become a standard procedure to improve the clinical utility of xenogeneic Abs by reducing human anti-murine Ab (HAMA) responses elicited in patients. However, CDR-grafted humanized Abs may still evoke anti-V region responses when administered in patients. To minimize anti-V region responses, the Ab may be humanized by grafting onto the human templates only the specificity-determining residues (SDRs), the residues that are essential for the surface complementarity of the Ab and its ligand. Typically, humanization of an Ab, whether by CDR or SDR grafting, involves the use of a single human template for the entire VL or VH domain of an Ab. We hypothesized, however, that the homology between the human template sequences and mAb to be humanized may be maximized by using templates from multiple human germline sequences corresponding to the different segments of the variable domain. This could be more advantageous in reducing the potential immunogenicity of the humanized Ab. This report describes the SDR grafting of the murine anti-carcinoembryonic antigen (CEA) mAb COL-1 using three different human germline V-kappa sequences as templates for the VL CDRs and another human template for the VL frameworks. In competition RIAs, the SDR-grafted COL-1 (HuCOL-1SDR) completely inhibited the binding of radiolabeled murine COL-1 (mCOL-1) to CEA, and showed that its binding affinity is comparable to that of the CDR-grafted Ab (HuCOL-1). The HuCOL-1SDR showed similar binding reactivity to the CEA expressed on the surface of a tumor cell line as the HuCOL-1. More importantly, compared to HuCOL-1 and the "abbreviated" CDR-grafted Ab, HuCOL-1SDR showed lower reactivity to patients' sera carrying anti-V region Abs to mCOL-1. HuCOL-1SDR, which shows a lower sera reactivity than that of the parental Abs while retaining its Ag-binding property, is a potentially useful clinical reagent. To the best of our knowledge, this is the first time a VL or VH domain of an Ab has been humanized by grafting the SDRs onto a human template comprised of several Ab sequences. We have shown that humanization of an Ab can be optimized using multiple human templates for a single variable domain of an Ab. This approach maximizes the homology between the target Ab and the human templates in both the frameworks and the CDRs by choosing as the template the human sequence that displays the highest local sequence identity to the frameworks and to each of the CDRs of the target Ab.

In vitro affinity maturation of a specificity-determining region-grafted humanized anticarcinoma antibody: isolation and characterization of minimally immunogenic high-affinity variants

PURPOSE

HuCC49V10 (V10), a humanized anticarcinoma monoclonal antibody (Ab) CC49, was generated by grafting only the specificity-determining regions (SDRs) of CC49 onto the variable light and variable heavy frameworks of the human Abs LEN and 21/28'CL, respectively. SDRs are those residues of the complementarity-determining regions that are most critical for antigen (Ag) binding. Compared with HuCC49, which was developed by conventional complementarity-determining region grafting, V10 has lower reactivity to the sera from patients who were previously given murine CC49 in clinical trials, although its Ag-binding affinity is 2-3-fold lower than that of HuCC49. To generate variants of V10 with higher Ag-binding affinity and lower sera reactivity, in vitro affinity maturation of V10 was carried out using phage display technique.

EXPERIMENTAL DESIGN

A limited library of Fabs was generated by replacing some of the SDRs with all possible residues located at the corresponding positions in human Abs. The library was enriched, by several rounds of panning, in Fabs that have high affinity for the TAG-72 Ag. The clones encoding the best binders were expressed in insect cells as whole Abs that were purified and characterized.

RESULTS

Competition radioimmunoassay and surface plasmon resonance measurements showed that two of the isolates, V14 and V15, have higher binding affinity than that of V10. In addition, the surface plasmon resonance analysis showed that the variants V14 and V15, compared with the parental V10, have lower reactivity to the anti-V region Abs using sera from patients who received murine CC49.

CONCLUSIONS

The two isolates, V14 and V15, which show higher Ag-binding reactivity and lower sera reactivity than the parental V10 Ab, are potentially more useful clinical reagents. These results demonstrate that phage display can be used to isolate variants of an Ab that are potentially less immunogenic in patients than the parental Ab from which they are derived.

Minimizing immunogenicity of the SDR-grafted humanized antibody CC49 by genetic manipulation of the framework residues

The murine mAb CC49 specifically recognizes a tumor-associated glycoprotein (TAG)-72, which is expressed on the majority of human carcinomas. This Ab has potential applications in the diagnosis and treatment of human carcinomas. However, patients receiving murine CC49 generate human anti-murine Ab (HAMA) responses, preventing repeated administration of the Ab for effective treatment. To minimize the HAMA response, two versions of humanized CC49 (HuCC49) were developed: (a) HuCC49 and (b) HuCC49V10 (V10). HuCC49 was developed by grafting the CC49 CDRs, while V10 was generated by grafting only the specificity determining residues (SDRs) of the CC49 onto the frameworks of the human Abs. During the generation of both HuCC49 and V10, a few murine framework residues that were believed to be essential for the integrity of the Ag-binding site were retained. However, the indispensability of these residues for the Ag-binding activity of CC49 has not been experimentally validated. In this study, an array of V10 variants were generated by replacing, by site-specific mutagenesis, the murine framework residues that were retained in the humanized Ab with their counterparts in the human templates. The variants were tested for their (a) Ag-binding activity and (b) reactivity to sera from patients who were previously administered murine CC49 in a clinical trial. One such variant, V59, compared to the parental V10, shows a significant decrease in its reactivity to the anti-variable region Abs present in the patients' sera, while it binds to the TAG-72 Ag with a slightly higher affinity. Variant 59, which is expected to be minimally immunogenic because of its low sera reactivity, is a potentially useful clinical reagent against human carcinomas. In this study, we show for the first time that experimental validation rather than reliance on the protein data bank (PDB) should be the criterion for the indispensability of framework residues for the humanization of any murine Ab to retain its Ag-binding property and reduce its immunogenicity in patients.

Grafting of "abbreviated" complementarity-determining regions containing specificity-determining residues essential for ligand contact to engineer a less immunogenic humanized monoclonal antibody

Murine mAb COL-1 reacts with carcinoembryonic Ag (CEA), expressed on a wide range of human carcinomas. In preclinical studies in animals and clinical trials in patients, murine COL-1 showed excellent tumor localization. To circumvent the problem of immunogenicity of the murine Ab in patients, a humanized COL-1 (HuCOL-1) was generated by grafting the complementarity-determining regions (CDRs) of COL-1 onto the frameworks of the variable light and variable heavy regions of human mAbs. To minimize anti-V region responses, a variant of HuCOL-1 was generated by grafting onto the human frameworks only the "abbreviated" CDRs, the stretches of CDR residues that contain the specificity-determining residues that are essential for the surface complementarity of the Ab and its ligand. In competition RIAs, the recombinant variant completely inhibited the binding of radiolabeled murine and humanized COL-1 to CEA. The HuCOL-1 and its variant showed no difference in their binding ability to the CEA expressed on the surface of a CEA-transduced tumor cell line. Compared with HuCOL-1, the HuCOL-1 variant showed lower reactivity to patients' sera carrying anti-V region Abs to COL-1. The final variant of the HuCOL-1, which retains its Ag-binding reactivity and shows significantly lower serum reactivity than that of the parental Ab, can serve as a prototype for the development of a potentially useful clinical reagent.

Carcinoembryonic antigen as a target for specific antitumor immunotherapy of head and neck cancer

Human carcinoembryonic antigen (CEA) is an oncofetal glycoprotein overexpression of which by gastrointestinal carcinomas is well known. Expression of CEA in head and neck cancer (HNC) is not widely recognized. It is important to note that most of these studies used polyclonal antibodies that may have cross-reactivity with CEA-related antigens. Currently, CEA is being evaluated in preclinical and clinical studies as a target for specific immunotherapy against gastrointestinal adenocarcinomas that express the antigen. This study was conducted to evaluate CEA as a potential target for specific immunotherapy against HNC. Immunohistochemical analysis of tumor tissue from 69 cases of squamous cell carcinoma (SCC) of the head and neck using a CEA-specific monoclonal antibody (COL-1) showed the majority to be positive for CEA. Tumor cell lines derived from human HNC were screened for CEA transcripts using nested reverse transcription-PCR. Constitutive expression of CEA mRNA was detected in 7 of 10 HNC lines. CEA protein was detectable in lysates from all 7 of the lines by quantitative fluoroimmunometry. SDS-PAGE/Western blot analysis of cell lysates from these lines showed a COL-1 immunoreactive product with a molecular weight equivalent to that of CEA. Cell surface expression of CEA was low for the SCC lines; however, there was moderate to strong cytoplasmic staining intensity for all of the CEA(+) HNC lines by immunocytochemistry. Additional supportive evidence for CEA as a target was demonstrated by the presence of cytolytic activity of an HLA-A2-restricted/CEA-epitope-specific human CTL against a CEA-overexpressing HNC-derived SCC line. These results suggest that CEA may be considered as a possible target for specific vaccine-mediated immunotherapy against HNCs.