Characterization of a V kappa family in Mus musculus castaneus: sequence analysis

To examine genetic variation at immunoglobulin (Ig) multigene loci over short spans of evolutionary time, we have compared members of an Ig kappa chain variable (V kappa) region family from several mouse species. In this study, seven unique Igk-V24 family members have been isolated from Mus m. castaneus and characterized by nucleotide sequence determination for comparison to their counterparts in Mus m. domesticus (BALB/c), and Mus pahari, representing 1-2 million years of evolution in the former case and 5-8 million years in the latter. Parsimony, together with evolutionary distances calculated for various pairs of Igk-V24 family coding regions, relate all family members to a common progenitor existing roughly 24 million years ago (Mya). A significant portion of the M. m. castaneus family consists of pseudogene segments in various degrees of progressive degeneration. The substitution patterns and divergence rates for all gene segments are characteristic of their respective subsets, especially in the areas flanking the coding regions. Complex and variable patterns of diversity are seen in potentially expressed coding regions, which appear to reflect quite different selective pressures on various subregions within the V kappa protein domain. These results indicate that evolutionary pressures are operating at the level of family subsets, their individual members, and subregions within similar molecules.

Exon-specific gene correction (conversion) during short evolutionary periods: homogenization in a two-gene family encoding the beta-chain constant region of the T-lymphocyte antigen receptor

The two genes encoding the beta-chain constant region of the T-lymphocyte antigen receptor appear to have undergone gene conversion in a number of species, including wild and laboratory mice. To examine the frequency of such events during short evolutionary periods, we have characterized the corresponding genes from an African pygmy mouse, Mus minutoides. Sequence analysis indicates that exon 1 regions from these genes have undergone conversion events independent of those observed in other mouse species. Furthermore, the conversion events in all murine species are limited to exon 1 sequences. One such event involves the insertion and subsequent transfer of an entire codon between the two genes. Comparisons with other murine C beta sequences suggest that gene conversion has occurred on the order of every 0.3 Myr during the evolution of a family consisting of only two genes.

Instability of immunoglobulin genes in S107 cell line

Somatic mutation occurs frequently in rearranged and expressed immunoglobulin variable region genes in vivo. In contrast, V region hypermutation seldom occurs in antibody-forming cells in culture. The S107 mouse myeloma cell line is one of the few cell lines that has been observed to generate V region mutations frequently and spontaneously in vitro. Detailed examination reveals that both the S107 tumor and the cell line derived from it contain and express a duplicated heavy-chain gene. In culture, only one of the two heavy-chain genes undergoes both V and C region mutation, and variants with complex phenotypes and genotypes arise as a result of mutation and segregation of these duplicated genes.

Expression of a class I MHC transgene: regulation by a tissue-specific negative regulatory DNA sequence element

In vivo patterns of expression of a miniature swine class I major histocompatibility gene, PD7, were analyzed both in situ in the pig, and in transgenic mice. Structural analysis of PD7 DNA sequences revealed that PD7 is highly homologous to the pig gene PD1, which encodes a classical transplantation antigen. Despite the extensive homology, PD7 is expressed in situ at markedly lower levels than PD1 in nearly all tissues. Introduction of PD7 into mice results in a pattern of PD7 expression in the transgenic animals that parallels that observed in situ in the pig. Comparison of two lines of PD7 transgenic mice, which differ only in the extent of 5' flanking sequence, reveals the presence of a silencer element. The silencer activity is tissue specific: differences in PD7 expression are observed only in lymphoid tissues and skin. Skin from both lines of transgenics mediates graft rejection, but the rate of rejection correlates with the level of PD7 expression.

Sequence and structure of Clp P, the proteolytic component of the ATP-dependent Clp protease of Escherichia coli

The ATP-dependent Clp protease of Escherichia coli contains two dissimilar components: the Clp A regulatory polypeptide, with two ATP binding sites and intrinsic ATPase activity, and the Clp P subunit, which contains the proteolytic active site. The DNA sequence of the clpP gene predicts a protein of 207 amino acids (Mr 21,679), which is in close agreement with the size determined by sodium dodecyl sulfate-gel electrophoresis of purified Clp P. Clp P has a native Mr of approximately 240,000, and electron micrographs of the protein show superimposed disk-like structures with a central cavity, similar in appearance to purified proteasomes from eukaryotic cells. Clp P is synthesized with a 14-amino acid leader which is rapidly cleaved in vivo to yield the 193-amino acid protein which has activity in vitro. The clpP gene is at 10 min on the E. coli map, close to that for the ATP-dependent Lon protease of E. coli and far from the gene for clpA. Primer extension experiments indicate that transcription initiates immediately upstream of the coding region for Clp P, with a major transcription start at 120 bases in front of the start of translation. Insertion mutations in clpP have been isolated and transferred to the chromosome; strains devoid of Clp P are viable in the presence or absence of Lon protease. Mutations in clpP stabilize the same Clp A-beta-galactosidase fusion protein specifically stabilized by clpA mutations, providing the first genetic evidence that Clp A and Clp P act together in vivo.

T cell derived IL-6 is differentially required for antigen-specific antibody secretion by primary and secondary B cells

IL-6 (formerly PCTGF, HP-1, BSF-2, HGF, IFN-beta 2, 26 kDa) is a recently defined lymphokine demonstrating activity on multiple cell types, including hepatocytes, thymocytes, T cells, plasmacytomas, and B cells. The biologic effects of IL-6 on lymphocytes, particularly B cells, suggest this factor may be involved in the regulation of normal immune responses. Accordingly, we have investigated the role of IL-6 in Ag-specific responses of B cells from both naive and Ag-primed mice. When Ag-primed splenic T cells were used as a source of help, naive (primary) B cell responses specific for the hemagglutinin molecule of the influenza A virus (PR8) were fully inhibited by the addition of an anti-IL-6 antiserum, and are thus IL-6 dependent. In contrast, secondary B cell responses were essentially IL-6 independent, being unaffected by this antiserum even at concentrations 10-fold higher than required to completely inhibit primary responses. This differential IL-6 requirement was further investigated by using a panel of hemagglutinin molecule-specific Th clones. Consistent with the above findings, a Th1 clone secreting biologically active IL-6 enables antibody secretion by both primary and secondary B cells, whereas Th1 clones that do not produce IL-6 support secondary responses, but fail to help primary B cell responses unless exogenous IL-6 is added. These results provide the first instance of differential lymphokine requirements among primary vs secondary B cell responses, and suggest T cell-derived IL-6 plays a critical role during the regulation of humoral immune responses. Moreover, functionally distinct Th1 clones were identified that differed in IL-6 secretion and their corresponding ability to induce Ig secretion by primary and secondary B cells.

T cell derived IL-6 is differentially required for antigen-specific antibody secretion by primary and secondary B cells

IL-6 (formerly PCTGF, HP-1, BSF-2, HGF, IFN-beta 2, 26 kDa) is a recently defined lymphokine demonstrating activity on multiple cell types, including hepatocytes, thymocytes, T cells, plasmacytomas, and B cells. The biologic effects of IL-6 on lymphocytes, particularly B cells, suggest this factor may be involved in the regulation of normal immune responses. Accordingly, we have investigated the role of IL-6 in Ag-specific responses of B cells from both naive and Ag-primed mice. When Ag-primed splenic T cells were used as a source of help, naive (primary) B cell responses specific for the hemagglutinin molecule of the influenza A virus (PR8) were fully inhibited by the addition of an anti-IL-6 antiserum, and are thus IL-6 dependent. In contrast, secondary B cell responses were essentially IL-6 independent, being unaffected by this antiserum even at concentrations 10-fold higher than required to completely inhibit primary responses. This differential IL-6 requirement was further investigated by using a panel of hemagglutinin molecule-specific Th clones. Consistent with the above findings, a Th1 clone secreting biologically active IL-6 enables antibody secretion by both primary and secondary B cells, whereas Th1 clones that do not produce IL-6 support secondary responses, but fail to help primary B cell responses unless exogenous IL-6 is added. These results provide the first instance of differential lymphokine requirements among primary vs secondary B cell responses, and suggest T cell-derived IL-6 plays a critical role during the regulation of humoral immune responses. Moreover, functionally distinct Th1 clones were identified that differed in IL-6 secretion and their corresponding ability to induce Ig secretion by primary and secondary B cells.

In vivo induction of IL-6 by administration of exogenous cytokines and detection of de novo serum levels of IL-6 in tumor-bearing mice

We investigated the capacity of several recombinant cytokines to induce IL-6 in vivo in both normal and tumor-bearing (TB) mice. Intravenous administration of human rhTNF-alpha, rhIL-1, rhIL-2, rhIFN-alpha A/D, and rmIFN-gamma were all capable of inducing circulating IL-6. rhTNF-alpha administration caused the greatest induction of IL-6. TB animals consistently produced more IL-6 in response to rhTNF-alpha than did normal mice (2 h after 4 micrograms rhTNF-alpha, TB = 24,100 HGF U/ml, non-TB = 3600 HGF U/ml of IL-6). A single daily i.v. dose of rhTNF-alpha (4 micrograms/mouse/day) for 5 days led to decreased IL-6 induction in TB animals by day 3 of treatment (peak levels of IL-6, day 1 = 72,800 HGF U/ml, day 3 = 23,400 HGF U/ml, day 5 = 26,400 HGF U/ml). rhIL-1 administration also resulted in considerable IL-6 production, although peak values were less than those resulting from administration of rhTNF-alpha. Administration of rhIL-1 induced similar IL-6 levels (TB = 10,025 and non-TB = 10,600 HGF U/ml) in TB and normal mice. Single high doses of rhIL-2, rhIFN-alpha A/D, and rmIFN-gamma induced lower but consistent levels of circulating IL-6 in mice with and without tumor. In addition, the sera of untreated TB mice contained levels of IL-6 which paralleled the extent of tumor burden (serum IL-6 in day 30 MCA 106 TB mice = 420 HGF U/ml). The detection of de novo IL-6 was also confirmed in animals bearing tumors of different histologies (the MCA 102 sarcoma, MCA 38 adenocarcinoma, and B16 melanoma). At no time was IL-6 measurable in the sera of untreated normal mice. The identification of IL-6 was verified by neutralization studies using specific antimurine IL-6 antibody. Although the exact role of IL-6 in TB animals remains to be elucidated, its known pleotrophic immune and metabolic effects may be important in the host response to malignancy.

In vivo induction of IL-6 by administration of exogenous cytokines and detection of de novo serum levels of IL-6 in tumor-bearing mice

We investigated the capacity of several recombinant cytokines to induce IL-6 in vivo in both normal and tumor-bearing (TB) mice. Intravenous administration of human rhTNF-alpha, rhIL-1, rhIL-2, rhIFN-alpha A/D, and rmIFN-gamma were all capable of inducing circulating IL-6. rhTNF-alpha administration caused the greatest induction of IL-6. TB animals consistently produced more IL-6 in response to rhTNF-alpha than did normal mice (2 h after 4 micrograms rhTNF-alpha, TB = 24,100 HGF U/ml, non-TB = 3600 HGF U/ml of IL-6). A single daily i.v. dose of rhTNF-alpha (4 micrograms/mouse/day) for 5 days led to decreased IL-6 induction in TB animals by day 3 of treatment (peak levels of IL-6, day 1 = 72,800 HGF U/ml, day 3 = 23,400 HGF U/ml, day 5 = 26,400 HGF U/ml). rhIL-1 administration also resulted in considerable IL-6 production, although peak values were less than those resulting from administration of rhTNF-alpha. Administration of rhIL-1 induced similar IL-6 levels (TB = 10,025 and non-TB = 10,600 HGF U/ml) in TB and normal mice. Single high doses of rhIL-2, rhIFN-alpha A/D, and rmIFN-gamma induced lower but consistent levels of circulating IL-6 in mice with and without tumor. In addition, the sera of untreated TB mice contained levels of IL-6 which paralleled the extent of tumor burden (serum IL-6 in day 30 MCA 106 TB mice = 420 HGF U/ml). The detection of de novo IL-6 was also confirmed in animals bearing tumors of different histologies (the MCA 102 sarcoma, MCA 38 adenocarcinoma, and B16 melanoma). At no time was IL-6 measurable in the sera of untreated normal mice. The identification of IL-6 was verified by neutralization studies using specific antimurine IL-6 antibody. Although the exact role of IL-6 in TB animals remains to be elucidated, its known pleotrophic immune and metabolic effects may be important in the host response to malignancy.

The murine Il-6 gene maps to the proximal region of chromosome 5

Murine Il-6 cDNAs were isolated by cross-hybridization with a human IL-6 cDNA from an IL-1 activated bone marrow stromal cell line (W20). Mouse-hamster somatic cell hybrids were utilized to localize murine Il-6 to chromosome 5. Genetic mapping with respect to En-2, AlbH, and Gus in backcross progeny from an interspecific mating between C57BL/6J and Mus spretus positioned Il-6 3 cM distal to En-2. The syntenic relationships of Il-6 and En-2 in mouse and man, as well as the potential role of IL-6 in tumorigenesis, are discussed.

The murine Il-6 gene maps to the proximal region of chromosome 5

Murine Il-6 cDNAs were isolated by cross-hybridization with a human IL-6 cDNA from an IL-1 activated bone marrow stromal cell line (W20). Mouse-hamster somatic cell hybrids were utilized to localize murine Il-6 to chromosome 5. Genetic mapping with respect to En-2, AlbH, and Gus in backcross progeny from an interspecific mating between C57BL/6J and Mus spretus positioned Il-6 3 cM distal to En-2. The syntenic relationships of Il-6 and En-2 in mouse and man, as well as the potential role of IL-6 in tumorigenesis, are discussed.

Purification and characterization of a newly identified growth factor specific for epithelial cells

A growth factor specific for epithelial cells was identified in conditioned medium of a human embryonic lung fibroblast cell line. The factor, provisionally termed keratinocyte growth factor (KGF) because of its predominant activity on this cell type, was purified to homogeneity by a combination of ultrafiltration, heparin-Sepharose affinity chromatography, and hydrophobic chromatography on a C4 reversed-phase HPLC column. KGF was both acid and heat labile and consisted of a single polypeptide chain of approximately 28 kDa. Purified KGF was a potent mitogen for epithelial cells, capable of stimulating DNA synthesis in quiescent BALB/MK epidermal keratinocytes by greater than 500-fold with activity detectable at 0.1 nM and maximal at 1.0 nM. Lack of mitogenic activity on either fibroblasts or endothelial cells indicated that KGF possessed a target cell specificity distinct from any previously characterized growth factor. Microsequencing revealed an amino-terminal sequence containing no significant homology to any known protein. The release of this growth factor by human embryonic fibroblasts raises the possibility that KGF may play a role in mesenchymal stimulation of normal epithelial cell proliferation.

The two-component, ATP-dependent Clp protease of Escherichia coli. Purification, cloning, and mutational analysis of the ATP-binding component

The ATP-binding component (Component II, hereafter referred to as ClpA) of a two-component, ATP-dependent protease from Escherichia coli has been purified to homogeneity. ClpA is a protein with subunit Mr 81,000. It has an intrinsic ATPase activity and activates degradation of protein substrates only in the presence of a second component (Component I, hereafter referred to as ClpP), Mg2+, and ATP. The amount of ClpA varies by less than a factor of 2 in cells grown in different media and at temperatures from 30 to 42 degrees C. ClpA does not appear to be a heat-shock protein since its synthesis is not dependent on htpR. Antibodies against purified ClpA were used to identify lambda transducing phage bearing the clpA gene. The cloned gene contains a DNA sequence expected to code for the first 28 amino acids of ClpA, which were determined by protein sequencing of purified ClpA. The clpA gene in the phage was mutated by insertion of delta kan defective transposons and the mutations were transferred to E. coli by homologous recombination. The clpA gene was mapped to 19 min on the E. coli chromosome. Mutant cells with insertions early in the gene produce no ClpA protein detectable in Western blots, and extracts of such mutant cells have no detectable ClpA activity. clpA- mutants grow well under all conditions tested and are not defective in turnover of proteins during nitrogen starvation nor in the turnover of such highly unstable proteins as the lambda proteins O, N, and cII, or the E. coli proteins SulA, RcsA, and glutamate dehydrogenase. The degradation of abnormal canavanine-containing proteins is defective in clpA mutants especially in cells that also have a lon- mutation. Extracts of clpA- lon- cells have ATP-dependent casein degrading activity.

Interaction and sequence diversity among T15 VH genes in CBA/J mice

Nucleotide sequences of the four genes composing the T15 heavy chain variable region (VH) family of the CBA/J mouse have been determined. Comparison of these sequences with their published BALB/c and C57BL/10 homologues reveals that nucleotide differences found between given alleles of two strains, i.e., CBA/J and BALB/c, are observed in other family members of the same strain. We suggest that these patterns of sequence variation are most readily explained by gene interaction (conversion). Additionally, the sequence of a CBA/J hybridoma, 6G6, proposed to have been generated by gene conversion, is directly encoded by the CBA/J V11 gene indicating that the putative conversion has occurred meiotically in the germline. These results are consistent with the premise that gene correction is occurring frequently among members of this family and that such processes may contribute significantly to the evolution of Ig variable region genes even in the relatively short time frame of inbred strain derivation.

Modern evolution of a single-copy gene: the immunoglobulin C kappa locus in wild mice

The immunoglobulin kappa light-chain constant region gene (C kappa) has been cloned and sequenced from five wild mouse species. Analysis of these data has permitted an assessment of single-copy gene evolution during a limited time period as defined by the genus Mus. Sequence conservation was found to be as high (or higher) in the 5' and enhancer regions as in the coding region. The pattern of substitutions throughout these genes suggests that parallel evolution has occurred frequently and that substitutions at replacement sites have not decreased significantly, owing to saturation during this period of approximately 10 Myr. Phylogenetic relationships have been determined among these wild species as well as among members of the genus Rattus.

Evolution of the IgA heavy chain gene in the genus Mus

To examine questions of immunoglobulin gene evolution, the IgA alpha heavy chain gene from Mus pahari, an evolutionarily distant relative to Mus musculus domesticus, was cloned and sequenced. The sequence, when compared to the IgA gene of BALB/c or human, demonstrated that the IgA gene is evolving in a mosaic fashion with the hinge region accumulating mutations most rapidly and the third domain at a considerably lower frequency. In spite of this pronounced accumulation of mutations, the hinge region appears to maintain the conformation of a random coil. A marked propensity to accumulate replacement over silent site changes in the coding regions was noted, as was a definite codon bias. The possibility that these two phenomena are interrelated is discussed.

Relationships between B cell and myeloid differentiation. Studies with a B lymphocyte progenitor line, HAFTL-1

A cell line, HAFTL-1, derived by in vitro transformation of fetal liver cells with v-Ha-ras, was found to have molecular and phenotypic characteristics of pro-B cells recently committed to the Ly-1+ B cell differentiation pathway. Stimulation of these cells with LPS resulted in their differentiation within either the B or myelomonocytic lineages. Thus, lines derived from LPS-stimulated HAFTL-1 cells were shown to be clonally related, as evidenced by common v-ras integrations, but to exhibit characteristics of pre-B cells (ThB expression, continuing DJ heavy chain rearrangements) or mature macrophages (expression of Mac-1 and Mac-2, lysozyme and nonspecific esterase production, phagocytosis) while maintaining their Ly-1+ phenotype. These results suggest that events resulting in the irrevocable commitment to a single lineage occur late in differentiation, at least within the pathway yielding Ly-1+ B cells and a proposed subpopulation of Ly-1+ monocytes and macrophages. Final commitment to these lineages is carefully orchestrated, as evidenced by restricted expression of Ly-5 isoforms and production of IgH transcripts.

Characterization of a new subfamily of class I genes in the H-2 complex of the mouse

A previously undescribed subfamily of mouse class I MHC genes, consisting of two to three members, has been identified. The structure and organization of one of these, Mb1, has been determined. Mb1 consists of five exons with open reading frames and potentially encodes a class I-like transmembrane protein. In the genome, Mb1 is linked to the H-2 complex, mapping telomeric to Qa. However, this gene has low (ca. 60%) nucleotide identity with other class I sequences and is no more related to mouse class I genes than to class I genes from other species. Mb1 transcripts have not been found in a variety of adult tissues or cell lines, suggesting that, if Mb1 is expressed, its expression is highly regulated. From DNA sequence identity and intron-exon organization, Mb1 appears to be a primordial gene which antedates mouse speciation and which has evolved independently of the rest of the class I gene family. Examination of various species of wild mice demonstrates the presence of a discrete Mb1 subfamily over long evolutionary periods of time.

Structure and expression of class I MHC genes in the miniature swine

The genome of the miniature swine, unlike other species, contains a relatively small class I MHC gene family, consisting of only seven members. This provides an excellent system in which to identify and characterize the regulatory mechanisms which operate to both coordinately and differentially regulate the expression of a multi-gene family. The structure of class I SLA genes, like other class I genes, consists of eight exons encoding a leader sequence, three extracytoplasmic domains, a transmembrane domain and intracytoplasmic domains. Despite the common structure, two sub-families of class I genes can be distinguished within the SLA family. One, containing the closely related PD1 and PD14 genes, encodes the classical transplantation antigens. Another contains the highly divergent PD6; the functions of the products of this subfamily, if any, are not known. The class I SLA genes share some common regulatory mechanisms, as evidenced by the fact that all three genes analyzed are transcribed in mouse L cells. Furthermore, interferon treatment of transfected mouse L cells enhances expression of all three genes. Both PD1 and PD6 are transcribed in vivo, where the highest levels of expression are observed in lymphoid tissues. Superimposed on the common patterns of class I gene expression are distinct ones, as evidenced by the findings that PD1 is preferentially expressed in B cells, whereas PD6 is preferentially expressed in T cells. These differences may reflect the extensive divergence of the 5' flanking sequences of these genes. Future studies will be aimed at elucidating the precise molecular interactions and mechanisms which give rise to the observed differential expression.

Amino acids at the site of V kappa-J kappa recombination not encoded by germline sequences

Murine V kappa-J kappa recombination is characterized by a maintenance of size at the site of recombination and the use of nucleic acids found only in germline sequences. This is in contrast to heavy chain VH-D-JH assembly where random nucleotides are added at the recombination sites to produce considerable size variation, even though the heptamer/nonomer recombination sequences are identical in both kappa and heavy chain genes. We have examined the origin of an unusual amino acid, Ile, found at the site of V kappa-J kappa recombination in antigalactan antibodies, by sequence analysis of the corresponding rearranged and germline genes. Results indicate that the Ile codon can be generated by use of a single nucleotide 3' of the V kappa segment in combination with the second and third nucleotides of the first codon of J kappa 5 or J kappa 4. However, several antigalactan antibodies express Ile in combination with J kappa 2. An Ile codon cannot be generated by recombination in any reading frame between germline V kappa and J kappa 2 segments. These results suggest that the origin of the Ile codon in lines using J kappa 2 may represent a novel even in murine light chain assembly, possibly similar to the de novo addition of nucleotides observed in heavy chain gene recombination.

Purification and NH2-terminal sequence of a plasmacytoma growth factor derived from the murine macrophage cell line P388D1

Plasmacytoma growth factor (PCT-GF), a putative macrophage-derived lymphokine essential for the in vitro viability and proliferation of early generation plasmacytomas, was purified from conditioned medium of the murine macrophage cell line P388D1. The purification of PCT-GF was accomplished by a batch concentration on trimethylsilyl-controlled pore glass beads, followed by: gel filtration chromatography; hydrophobic interaction HPLC; and reverse-phase HPLC. SDS-PAGE analysis of the purified PCT-GF revealed a single band of Mr 23,000. The amino terminal sequence of PCT-GF was established as NH2-Pro-Thr-Ser-Gln-Val-Arg-Arg-Gly-Asp-Phe-Thr-Glu-Asp-Thr-Thr-Pro-Asn- Arg-Pro-Val-Tyr-Thr. No significant homology was found between this sequence and proteins in the National Biomedical Research Foundation database, suggesting that PCT-GF is a new lymphokine unrelated to previously described growth and differentiation factors.

Purification and NH2-terminal sequence of a plasmacytoma growth factor derived from the murine macrophage cell line P388D1

Plasmacytoma growth factor (PCT-GF), a putative macrophage-derived lymphokine essential for the in vitro viability and proliferation of early generation plasmacytomas, was purified from conditioned medium of the murine macrophage cell line P388D1. The purification of PCT-GF was accomplished by a batch concentration on trimethylsilyl-controlled pore glass beads, followed by: gel filtration chromatography; hydrophobic interaction HPLC; and reverse-phase HPLC. SDS-PAGE analysis of the purified PCT-GF revealed a single band of Mr 23,000. The amino terminal sequence of PCT-GF was established as NH2-Pro-Thr-Ser-Gln-Val-Arg-Arg-Gly-Asp-Phe-Thr-Glu-Asp-Thr-Thr-Pro-Asn- Arg-Pro-Val-Tyr-Thr. No significant homology was found between this sequence and proteins in the National Biomedical Research Foundation database, suggesting that PCT-GF is a new lymphokine unrelated to previously described growth and differentiation factors.