1. Introduction
B lymphocytes initiate their development in the bone marrow and complete maturation in peripheral lymphoid organs [
1]. Cell development starts with pre-progenitor(pro)-B cells and undergoes through pro-B and pre-B cells to immature and mature B lymphocytes expressing B cell receptors (BCR). Later on, B cells express antibodies (immunoglobulins) and then develop into larger plasma cells, which are specialized in antibody secretion [
2]. While antibodies are crucial during the immune response to many diseases, some antibodies recognize polypeptides naturally expressed by the normal cells in the body resulting in various autoimmune diseases [
3].
Early stage B cell maturation is associated with the V(D)J recombination process, when Variable (V), Diversity (D), and Joining (J) gene segments assemble through DNA recombination resulting in immunoglobulin heavy (IgH) and light (IgL) chains of antibodies [
1,
4,
5]. The V(D)J recombination process depends on the DNA double-strand breaks (DSBs) initiated by recombination-activating gene (RAG)1/2 and then repaired in an error-prone manner by the non-homologous DNA end-joining (NHEJ) molecular pathway. The NHEJ is initiated by Ku70/Ku80(Ku86) factors recruited to the DSB and then facilitating recruitment of downstream proteins, such as DNA-dependent protein kinase, catalytic subunit (DNA-PKcs), DNA ligase 4 (LIGIV), X-ray repair cross-complementing protein 4 (XRCC4), XRCC4-like factor (XLF), Paralogue of XRCC4 and XLF (PAXX), and Modulator of Retrovirus Infection (MRI) [
5,
6]. Mature B cells can change the constant region of immunoglobulins in another recombination process called class switch recombination (CSR). The CSR is initiated by DNA lesions introduced by activation-induced cytidine deaminase (AID), and DNA breaks are repaired by NHEJ [
1,
4,
5,
6].
There are several strategies used to treat autoimmune diseases, including targeting the antibody-expressing B cells and surgically removing peripheral lymphoid organs [
3], although these methods are not ideal and patient management needs further improvement. One strategy is to target antibody-producing B cells (plasma cells) via enzymes specifically expressed in large amounts. One such protein is leukocyte tyrosine kinase (LTK) [
7,
8]. It was recently suggested that LTK-positive cancer cells as well as plasma cell-mediated diseases can be treated using tyrosine kinase inhibitors [
9].
LTK was identified as an endoplasmic reticulum (ER)-bound protein required for efficient secretion (including antibodies), and it was proposed to be a potential target for the development of new medicines [
7]. LTK localizes to the ER and regulates ER export [
7]. The LTK is a tyrosine kinase, which is very similar in structure to the anaplastic lymphoma kinase (ALK) [
7]. The ALK kinase is a known target in cancer therapy [
10], and the available medicines targeting ALK might also target the LTK.
Drug repurposing is an important direction in using developing medicines to treat new diseases by targeting the same or different pathways [
11,
12,
13]. While a lot of drug repurposing research was made focusing on anti-viral drug treatments and combinations [
14,
15,
16,
17,
18,
19,
20,
21,
22], here, we focused on drugs approved to treat ALK-positive cancers (alectinib, ceritinib, crizotinib, brigatinib, entrectinib, lorlatinib [
10]) to target ALK-negative but LTK-positive mature B cells.
Gene fusions are an important driver of oncogenesis, and their detection has improved patient outcomes. However, oncogenic drivers remain unknown in a substantial proportion of lung cancers [
23]. Examples of genes participating in oncogenic fusion include ALK, ROS1, and RET [
24]. CLIP1-LTK fusion is a recently-reported translocation associated with lung cancer [
25]. Because of similarity between ALK and LTK structure, it was possible to use medicines previously approved to treat ALK-positive lung cancers to target LTK-positive ones [
25]. In particular, five FDA-approved drugs (alectinib, brigatinib, ceritinib, crizotinib, and lorlatinib) as well as multikinase inhibitors entrectinib and repotrectinib, were efficient in targeting LTK-positive lung cancer cells [
25]. As B lymphocytes are known to express LTK (but not ALK), we proposed that the drugs listed above could target B cells that would allow drug repurposing to treat B-cell-related disorders [
9].
Here, we exposed human B cells to tyrosine kinase inhibitors and found that alectinib, brigatinib, ceritinib, crizotinib, and entrectinib inhibit the growth of B lymphocytes. Furthermore, we validated the findings using two alternative B-cell stimulation systems. Pre-clinical studies involving in vivo models will be needed to find out more optimal medicines and concentrations for patient treatment.
4. Discussion
Antibody-expressing B lymphocytes, including a more mature form, plasma cells, are therapeutic targets during severe autoimmune disorders, e.g. immune thrombocytopenia [
3]. The treatment options include corticosteroids, rituximab targeting the majority of B cell populations expressing CD20, daratumumab targeting mature B cells expressing CD38, and sometimes splenectomy [
3]. New and more effective methods of antibody-expressing B cells targeting are required.
Here, we tested several known tyrosine kinase inhibitors developed and approved to inhibit ALK-expressing cells. We used these inhibitors, however, to target ALK-negative and LTK-positive plasma cells that secrete a lot of proteins (antibodies) using ER [
7]. The populations of BLIMP1/IRF4 double-positive plasma cells were reduced correlating with inhibitor concentrations (
Figure 2 and
Figure 3). Nevertheless, BLIMP1/IRF4-negative B cells, previously selected as CD19-positive, were alive (
Figure 2 and
Figure 3), suggesting some level of specificity against antibody-secreting cells.
We used the tyrosine kinase inhibitor concentrations available in literature [
25] to optimize the protocols and to find suitable conditions for our experiments (
Figure 1). The radioactivity assay based on the accumulation of 3H Thymidine was used for decades in different variations. However, it was criticised by several authors due to several limitations, i.e. irradiation of the cell components by the radioactivity itself [
34]. Nevertheless, this assay is still widely used (e.g. [
32,
35,
36,
37]), in combination with other methods. Thus, we also used two flow cytometry-based methods to analyze the effects of tyrosine kinase inhibitors on stimulated B cells (
Figure 2 and
Figure 3).
ALK and LTK are structurally similar protein tyrosine kinases, involved in various processes, including oncogenic transformations and autoimmune disorders [
38]. Recently, CLIP1-LTK fusion was identified as a driver of non-small cell lung cancer (NSCLC) [
25]. While ALK is a well-established translocation partner in lung cancer, only initial knowledge is available regarding LTK-dependent lung cancers [
23,
25]. Similar to ALK-positive patients, some of the LTK-positive lung cancer patients had metastases to the central nervous system, and the smoking history in LTK-positive patients was variable [
23,
25]. Identification of LTK as a marker and target for lung cancer patients has several implications, including development of new drugs specifically targeting LTK, which in turn can boost the therapy of other LTK-positive pathologies. All the drugs used in our current study, i.e. alectinib, brigatinib, ceritinib, crizotinib, entrectinib, and lorlatinib, were originally developed to target ALK, and appeared to reduce LTK phosphorylation in the context of CLIP1-LTK [
25]. However, lorlatinib was selected as the best drug against CLIP1-LTK-induced cancer. Contrary, in our studies, lorlatinib had the lowest effect on B cell proliferation (
Figure 1).
Previously, we demonstrated that multiple drugs can be reused as potential antivirals [
11,
12,
13,
14,
15,
16,
17,
18,
19,
39,
40,
41,
42,
43]. Our current study is the first one to demonstrate that ALK inhibitors are acting on ALK-negative but LTK-positive [
8] plasma cells. It opens options for further drug repurposing, potentially considering all available inhibitors against ALK to be used against LTK-positive cells, and, in particular, in cases of B-cell dependent disorders, such as autoimmune diseases (e.g., immune thrombocytopenia) or cancer (e.g., multiple myeloma).
Moreover, during the B lymphocyte development, in the processes known as the V(D)J recombination and class switch recombination, there are many cases of known genetic interaction [
4,
5,
6]. Previously, synthetic lethality between the DNA repair proteins Poly(ADP-Ribose) Polymerase 1 (PARP1) and Breast cancer type 1 (BRCA1) resulted in the development of an anti-cancer drug, olaparib [
44]. Today, genetic interaction is known in B lymphocytes between NHEJ factors and DNA damage response proteins (DDRs), for example, between XLF and Ataxia telangiectasia mutated (ATM), histone H2AX [
45], a mediator of DNA damage checkpoint protein 1 (MDC1) [
29], p53-binding factor 1 (53BP1) [
46], PAXX [
31], and MRI [
47]. Thus, it is also possible to look for the B lymphocyte targeting options using known and to be discovered genetic interactions, including synthetic lethality, in the pathways required for B cell maturation, including the V(D)J recombination and class switching.
During the first screening (
Figure 1), we identified several tyrosine kinase inhibitors resulting in reduced rate of proliferation of B cell populations, including alectinib, brigatinib, ceritinib, crizotinib, and entrectinib. To validate the findings using two other methods, we focused on ceritinib and entrectinib. However, alectinib, brigatinib, and crizotinib are also good candidates to be tested in the following studies, in addition to multiple similar tyrosine kinase inhibitors available and reported in the literature (e.g., [
25]). The cells respond to ceritinib (3 µM) differently in different experimental settings (
Figure 2 and
Figure 3). However, it is hard to compare these data points because the experiments were performed using samples from different donors and the timing of the experiments differs (seven days for
Figure 2 and twelve days for
Figure 3). In both cases, ceritinib had a clear effect on B cell populations, validating the data in
Figure 1.
The populations of the cells presented in
Figure 2 and
Figure 3 are not identical. We explain the different shapes of the cell populations for the following reasons. First, the stimulation cocktail used was different, i.e. as described in the Materials and Methods for
Figure 2, and a commercially available kit (ImmunoCult) for
Figure 3. Moreover, the cells described in
Figure 2 were incubated for five days before the inhibitors were added for another two days. Differently, the cells in
Figure 3 were incubated ten days before the inhibitors were added. The cell fixation and population gating procedures, nevertheless, were the same for both types of experiments (Materials and Methods, and
Supplementary Figures S1 and S2). In particular, dead cells and debris were always excluded from the analyses, and each data point represents an individual donor.
In this study, we focused on tyrosine kinase inhibitors targeting human B lymphocytes. However, it is possible that the same inhibitors will be targeting other cell types, e.g. T lymphocytes, fibroblasts, etc. It cannot be excluded, because LTK can be expressed by various cell types, and because the inhibitors are promiscuous and can potentially target different enzymes, including in B cells. What is clear, however, is that these inhibitors do not inhibit proliferation of all the cells (
Figure 2 and
Figure 3), but rather some cell populations are more sensitive (e.g., BLIMP1+IRF+) than others (e.g., BLIMP1-IRF-).
Further studies will include broader use of ALK inhibitors against LTK-positive cells, and potentially vice versa. Following the cell-based experiments, pre-clinical studies will be done on animal models. However, the drugs are already approved to be used in human patients, although to treat different diseases, which is why future approval of the treatment options would be simpler.